Well here's some useless stats I just cooked up which are hard to make sense of but seem to follow a trend. There are 4 sets of chrono data found in this thread:
http://www.airsoftcanada.com/showthread.php?t=51752 on pages 1 (middle), 3 and two on 5 compliments of Stalker and m102404 all using .2/.25/.25/.3 Bastards. I calculated all the energies from each individual fps reading, took averages of those energy values in for each weight (in each of the 4 sets of data) then calculated the percentage increase/decrease in energy (compared to .2 as a standard). Here's what it looks like:
Page...... 0.25....... 0.28....... 0.30
1........... 5.910%... -1.752%... 5.266%
3........... 3.388%... -2.792%... 2.433%
5.1........ 9.008%... 2.629%.... 8.815%
5.2........ 2.448%... -8.259%... 1.421%
Avg:...... 5.189%... -2.543%... 4.484%
Note for m102404's results I ignored the outliers (used the 10 middle values).
The trend is for .25's and .30's to actually increase in enegy and for .28's to actually decrease for some reason.