A BN2T pilot from Hampshire, not police ASU at Lee are you by any chance? If so, I wrote the flight test plans and most of the clearance report for your Islander (although as it happens wasn't available for testing week so have never actually flown in the beast).
Anyhow for smaller aircraft like yours, it's acceptable and usual practice to reduce all climb perf data to density altitude, then for actual performance convert to local conditions. In other words, we assume that at a given density altitude the climb (and take-off) performance will be identical.
Given that, obviously an increased pressure setting on your altitimeter, will apparently give better climb performance at a given pressure altitude (since density altitude will be better assuming all else is equal). Similarly at ISA- temperatures you'll get better climb performance for a given pressure altitude, and at ISA+ temperatures you'll get poorer climb performance as the air density drops off.
You'll find this covered quite well by the BN2T operators manual, figures 5-12 to 5-16.
If you need performance data over a reasonable height bracket, it's normal to do matched pairs of performance climbs, into and downwind, which does a reasonable job of removing variation due to local conditions - although to be frank so long as you are 50 miles or so from the nearest front throughout the test, it's probably nugatory effort.
All aircraft performance will be better in still than in turbulent air (except possibly glide performance which may be better if the turbulence is primarily thermic), essentially because of the efficiency advantages of steady state conditions. Just compare GPS groundspeed on a long cruise leg, at constant IAS / Alt you'll lose anything up to 10% of groundspeed in moderate turbulence. I've never seen an actual analysis of loss of climb performance due to turbulence, so couldn't guess what magnitude of loss you'll see.
As to making the figures looking better, any grown-up pilot or engineer assessing an aircraft will be standardising to ISA / fixed weight conditions in any analysis they do (or comparing actual performance to published data at same conditions). But, if somebody is idiot enough not to do this, then clearly lightweight, cold, high pressure setting, nil turbulence will give better apparent results. And even more obviously, a good headwind component will give a better apparent climb gradient.
If you are looking at Islander performance data, virtually all of B-N's performance testing is done to the SW of the IOW which, with the usual prevailing wind, means no obstacles for about 7000 miles or so, and hence very little turbulence. So, real-world performance over land, particularly in summer flying conditions, will routinely be a little poorer.
Looking at your profile you surprise me with the question slightly, all this is covered pretty well by the ATPL(A) syllabus isn't it?
G