Been there done that
First find out if your jet is too lean (you probably are)
http://www.vansairforce.com/community/showthread.php?t=20875&highlight=high+egt&page=2
http://www.vansairforce.com/community/showthread.php?t=18000&highlight=jet
Don't waste your money on the 666 jet kit (for 1960's Mooney's). Yes richer but often still not enough, it cost a lot of money and drilling your jet is easy and fun (and cheaper) and works. I know folks who bought the +$200 kit and ended up still drilling it out. You don't don't need a spare jet; they cost too much and they don't wear out.
http://www.vansairforce.com/community/showthread.php?t=30111&highlight=drill+jet
http://www.vansairforce.com/community/showthread.php?t=6246&highlight=lean
How To:
Pull the main jet out (remove bowl, pry jet lock tab down and unscrew). The jet is right there, a brass straw angled down between the floats. Hand fit the largest # drill bit you can, than drill one or two # size larger (we are talking thousands of an inch). When you drill the jet use some dry drill lube and high RPM. You can use a hand electric drill. It does not have to be air-drill fast. Don't go crazy going back and forth. You don't need a drill Jig since you are really reaming up an existing tube. Drill one size up at a time. Don't go to final size, which as I say is usually 1 to 3 sizes higher than the stock size.
Clean-up burr or sharp corner (don't go crazy radius-ing or chamfering). You may or will needs some parts. Bowl lock washers, main jet lock washer, main bowl gasket. Some times you can get by with reusing parts like gasket if the carb is newer. If an older carb plan on replacing parts. Sometime a lock tab is good for one bend, but I would not do it, especially the jet lock tab washer. The jet probably will not fall out but you don't want a piece of metal floating around / sitting on the bottom of the bowl. Clean the jet and reassemble. Don't forget to not where the cables attached to what hole. Its easy to put them in the wrong hole.
It usually takes two iterations to get it right. Its easier to drill up in small steps than go too big at first. BTW people have repaired their jet by brazing the hole closed and than re drilling. I would just say go in small steps, even if it means taking your jet out twice.
This is not an exact science but not rocket science either. The jet is just a brass fitting with a whole in it, albeit a calibrated hole. Once you drill it out, put it together and fly the "test" and note EGT. You are looking for the ability to have at least 150F ROP at WOT at 8000ft. 175-200F ROP is OK or actually good. However more than that is too rich.
You first iteration oversizing the jet ALL DEPENDS ON HOW LEAN YOU ARE.
My old O-320 RV-4 was at almost Peak EGT at WOT, 8,000' (+75% power). Any movement of the mixture control caused the EGT to almost drop immediately. That is typical of RV's. Also my plugs where very light. I knew I was very lean. Drilling allowed me to lean to peak at 8k WOT and set econ or max power (100-150F ROP). With full rich in the WOT 8K condition you want about 175F ROP. I was able to only get about 150-160F with full rich, still a tad lean but not bad, so I stopped there. Remember at full power you need that extra rich mixture you see at 75% cruise. So ROP with a margin of 175F ROP is pretty good number.
Part of the lean issue at 8k is the RAM air affect in cruise. I rationalized that my 150F ROP (should be 175-200F) with full rich was OK, since on takeoff there is no RAM air (or a lot less). At sea-level and cool temps I was probably still a little lean on initital takeoff with 100% power. However I throttle back by 1000ft and all is well. The issue with 100% power and not having sufficent FF is you lose a little power but also some detination margin. If running AUTO gas you don't don't want to be lean since it has lower octan.
On my RV-4 I did notice a slight increase in performance. I had a CS prop so didn't notice increase in RPM of course, but for fixed prop RV's you'll no doubt get a little more RPM due to increase in power from richer mixture on takeoff. I did notice lower CHT's in summer and the plugs looked normal with a light carmal color.
You can get TOO rich. If mildly over rich, you'll just use a tad more gas at full power (a big deal now). Your gas milage depends on your judicious use of those little black and red knobs, lean or rich jet. Of course if you go crazy and over oversize the jet, you can loose power and cause engine grief from engine deposits on valves, pistons and fouled plugs. Not good either. Too lean can cause engine damage in the form of burned valves, loss of power or worse, detonation, which can cause serious damage to piston, crank and lead to engine failure.
Don't be afraid its a neccesary evil of the RV's. Factory planes that are slower, have more restiricve intake and exhaust just don't need a jet as rich as we do. If you look at all the installation loss of a typical C-172, say with a claimed 160HP, it might only get 140HP max! Than add 20% loss for the prop efficency, the C-172 might only has about 112HP at the prop. Aftermarket exhaust makers and cowl makers can improve slower factory planes. One after market manufacture claims an increase of 15HP for their exahust. That does not make a 160 HP engine 175HP, it just gets back all the restrictive loss of the stock pipes closer to the rated 160HP (on the test stand).