VAN's provides the following table for prop selection, specifically for Sensenich props:
............Aircraft...........Horsepower.....Required Pitch
RV-3.....................150/160..............(81)
RV-4.......................150...................(79)
RV-4.......................160...................(81)
RV-6/7/8..................150..................(78)
RV-6/7/8..................160..................(80)
RV-6A/7A/8A/9A......150..................(77)
RV-6A/7A/8A/9A......160..................(79)
RV-4.......................180..................(85)
RV-6/7/8..................180..................(85)
RV-6A/7A/8A...........180..................(85)
You say you have a 78" pitch, which may not be the optimum selection for your engine/plane combo, according to VAN's.
I had the 150 HP RV6 with the 78" pitch and my top speed WOT was about 165 knots with all of the fairings tight and the newer style wheel pants. (They do make a difference. I know, because I switched from the older style after about 500 hrs on the -6.)
One thing that happened to me, I had a stuck exhaust valve and didn't know it. I did notice a drop in performance but the engine seemed to run fine, etc. When I finally figured it out and fixed the bad cylinder, my performance went back up to normal.
My point is, it could be any one of a number of things, or a combination of them, that is stealing your power. I agree with all of the previous posts in that any one or several of them could be a problem.
Start with the engine. Check out everything you can: clean air filter, good plugs, timing, harnesses, compression in all cylinders, verify fuel flow...check that the fuel pump is pumping properly, verify mixture and throttle controls are all working to their limits, if you have a primer system, make sure it isn't leaking, look for induction leaks, etc.
Then, start looking at the fairings...are they all snug and tight or do they have gaps and open areas that create drag? Make sure the intersection fairings at the gear legs and fuselage are staying tightly up agasinst the fuselage. I have seen them open up in flight, but close back up on the ground. The RVs are fast anyway, but fit and finish does make a difference.
Try to verify that all of your instrument readings are accurate. Looks like you are already doing that. One easy check, in doing a series of stalls, see ifyour indicated airspeed is around 55 MPH...if so, that's at least one point of reference for the IAS; however, that doesn't necessarily mean it is correct at other higher speeds. You can check out you pitot and static system to make sure they are open and don't have leaks, Verify that the pitot tube is straight into the relative wind. Make sure the static ports are not clogged.
So, there are a lot of things you can do to check and tweak your plane before you go to the extreme of trying a new prop. If possible, borrow one from someone with an RV who is willing to help. You might even try the prop manufacturers. Sometines they will loan you one to try, if they have one available.
Good luck...I think you will eventually get it all straightened out and have a good time doing it, even though it is probably a little frustrating for you now. In the end, you will learn a great deal about your plane that you probably didn't know before.