Hey robbo,
I presume you are referencing speed / FPS.
The only way that I can help you is through my results.
Simply choose the straight or tapered band cut, and then compare the 6mm FTS for 0.45 and 0.40.
For example, if you wanted to compare 3/4 - 3/8, then 0.45 shows 280 FTS, whereas 0.40 shows 270 FTS.
So in this case ONLY, 0.45 has the edge for FPS.
You can simply compare any results you want side by side, for the band cut and ammo size that you are interested in.