There are a lot of factors to be taken into consideration when fitting mods to a/c and then perf. checking;ie, how long since the previous perf check,; deterioration/improvement maybe because an engine/prop. may have been changed; same pilot doing the tests, using his technique; instrument calibration recently( including flight instruments). As JF says, before any mod , which may look similar to previous equipment, it is always adviseable to do a datum check so you know what the change has been.
If these back-to-back tests can be carried in the same time-frame,and weather conditions, then you will have a good comparison.
I have spoken to a forecaster who says it is very difficult to be precise about the effect of a low/high pressure system with respect to airmass rising/falling rates as it is dependent on too many factors,ie smoke and mirrors magic,isobars,temperature at upper and lower levels etc,etc., although actual rates are fairly low.That said, if previous climbs have been reduced correctly, then you should have a good basis for comparison.
What you can do, however is to do a series of "partial climbs" at slightly differing airspeeds. ie if normal climb speed is 90 kts, then try climbs at 95, and 85 kts as well. It should give you a spread of performance. BEWARE,if you are looking at s/e perf, as you may be getting close to Vmca! The figures are only illustrative, so you use the POH.
I presume its a turbine you fly , so you may find that a slightly faster speed is beneficial.
