What does 150 knots mean? 150 KIAS, KTAS, KTGS? Has the pitot-static system in the plane been calibrated?
Also, what does "at altitude" mean? Pressure altitude, Density altitude, 7,500' MSL, or 11,500' MSL?
What engine? What prop? What power setting? What fuel flow?
Way to many variables and unknowns to even begin to answer the question.
IMHO, rigging will be a very minor issue, if one at all.
The definition of calibrated airspeed is indicated airspeed corrected for installation errors. In my case, my AS indicator was in error by 5 knots. I thought it was my static port installation until I bought a couple of G5's and that solved the problem.
Just because the static system has had its 24 month inspection, it does not mean your airspeed indicator is accurate.
"At altitude" usually means 7000 feet or above because this is where the engine power can be set at full throttle. Most manufactures set max cruise at an altitude of around 7000 feet because this will give you 75% power, maximum cruise speed and less dense air. Most engine manufacturers will not recommend cruise powers above 75%.
The only way to determine your max cruise true airspeed is to climb to 7000 feet and fly two directions 180 degrees difference and note your ground speed then add them together and divide by 2. I have tried flying up to 4 directions 90 degrees apart and came up with a difference of less than 1 knot. Probably more accurate but not worth the effort.
150-155 Knots seems about right for a 0320 with fixed pitch prop. You could check for drooping control surfaces. You can raise the ailerons and get a few more knots of speed but once the plane is built, not much can you do with rigging because unlike some certified airplanes, they don't have any adjustments. Wheel pants, leg fairings and cooling drag are the major areas of improvement for the average RV IMO but take a substantial amount of work for the results.
If I am on a cross country of more than 150 miles, I will usually fly at above 10K feet because that gives me 179 knots TAS and less than 7 GPH in my 0360 180HP Superior with fuel injection and electronic ignition.
Getting to excited about cruise probably is not worth the effort. I was at Oceano, CA the other day flying to Fresno with a friend in a 182 Cessna. I took off probably 5-10 minutes after he did and passed him about half way. I climbed to 8500 and he stayed down at 6500. I was putting my plane away in the hanger when he landed. Other than burning a lot less gas, the time saved was insignificant.