Every little bit helps
There are a few choices with the wing, mostly lighting and pitot, as well as priming internal parts.
Every little bit helps, but the big weight items are the prop, engine and than accessories (panel, light, interior and exterior)
From tip to tail:
Prop: wood (RV-7's suffer from tail heaviness so metal props not such a liability CG wise, RV-9? I don't know?)
Engine: (I)O320, with RV-9, heavier 360's are not really an approved option.
Engine instrumentation: CHT, EGT on all cylinders and Fuel Flow is optional. I think its probably weight worth spending, but many planes have flown for many decades (cessna, piper, etc) with out these items monitored. Leaving it off saves, weight, cost and build time. How about just one or two CHT/EGT channels. Flying GA planes in the 80's I can't remember many that had more than one CHT or EGT if any. We seemed to get along fine with out engine monitors. With that said with a well balanced Fuel Injection (FI) and careful leaning, you can run Lean of Peak (LOP), saving some gas. A 4x4 channel engine monitor is key to getting safe LOP operations. If you are carburated than LOP is pretty hard to impossible to get regardless of engine instruments, so standard roughness or 75-150F ROP ops works as well as it always has, therefore a single CHT/EGT may be good enough. Again I love the 4x4 engine monitor and its a great tool.
Cowl: Finishings and filling pin holes, read about it, search these archives for threads. The idea is to fill with min weight. You can add lots of weight with excessive filler.
Lighting: Nav lights, strobes, landing light...... LED Nav lights could save some weight. For night two all in one (nav/strobe/nav) wing tip units mounted externally is probably best. If you go with lots of lights, landing light and tail light/strobe, you may need a larger alternator than a smaller (lighter) one. Each light adds wiring, fuse/CB and switch for each light cuircuit. Also running wire out to wing tips and tail is not totally trivial. Landing lights are not needed even at night. How much night are you going to do if any? Do you need two landing lights or one or none?
Heated Pitot: My "Pet Peeve" on VFR planes with VFR pilots. This one item adds big cost and yes weight. Heavy current demand adds heavy wires run out the wings and demands a bigger alternator. If you can hold back from this popular "must have" you can save some weight. Do your really need it, even if you plan on occasional IFR?
Panel & Electrical: Some obvious weight to be saved with autopilots, dual EFIS and panel mounted entertainment systems. I am a circuit breaker (CB) man, but the popular use of an automotive plastic fuse block and blade fuses looks lighter than a bank of CB's.
On the other hand you don't NEED a fuse or CB for each item.
Builders today tend to put in 20-30 separate fuse's or CB's in. You can gang several items on one fuse or CB if the total load does not exceed the capacity of the fuse or CB and the wire is protected. The key is the wire gage, which is what you are protecting, not the individual device. For example 10 amp protection for sevral devices, all with 18 awg feed wire, is fine, for example, five 2-amp items or three 3 amp items. Look at Vans planes, they have a hand full of CB's for the whole plane and that is it. The last few Van prototypes I saw actually had a few CB's or a few fuses mounted visibly in the panel. Van's factory planes tend to be very light weight when compared to average builder weights and this is one reason, simple, minimal, functional. Bottom line, the popular electrical design books like Aeroelectic (by Bob Nuckolls) tend to be very involved and promote dual battery, dual alternators, multiple buses, diodes and more switches. There is good stuff here but KISS - Keep It Simple Stupid, saves weight. If you have an all electric aircraft for IFR flight or an electrically depended engine, than redundancy of the electrical system is needed, so the weight of two batteries or alternators may be justified. If you are a day / occasional night VFR pilot with a magneto fired, mechanical fuel pump Lyc with basic instruments, than you don't need that Boeing 747 electrical system. Is it ok to put 30 fuses in that VFR plane? Yes, but it adds weight.
Vacuum system: Pretty much gone the way of the DoDo bird. Leaving it out will save lots of weight. Of course that puts the emphasis on the electrical system with electrically dependant flight and engine instruments.
Interior: Bare, painted upholstered, except covered seat cushions. It does not have to be horrible or crude, just spartan.
Paint and Priming: If you prime all parts inside and out, it will add weight. That is a fact. How how much weight is debatable, but it does add weight. If you do prime use "wash primer" which is translucent. In fact its reported widely that Van does not PRIME his factory planes, at least in the past. Van just etched, alodine and painted the exterior. Why? Weight. If you leave it un-primed and unpainted you can save $4000-$8000 and up to 30 lbs. How long will it last? Depends on many factors but probably will out live you.
Everything: Every little nylon tie wrap, nut, bolt, washer, clamp, bracket adds weight. Consider using electrical lace tape verses nylon ties.
Now some weight is good for your comfort, safety (fire extinguisher) or type of operation (big dual landing lights for night ops off of dark private fields). One basic thing is build it per plans. Anything you add or change will usually add one or more of the following: time, money and almost always weight.