Since I haven't opened the Incredible 2, I don't want to say one way or the other if you can make a 1500 fit inside. I'll let you take that risk yourself. However, if I do get the chance to try it myself, I will certainly report back my findings. Also, while the HTC OEM does fit inside the original Incredible, it is marginally thicker than the 1300. This makes it harder to firmly clamp the back on properly, and it makes the back creek a little when pressed in certain spots. Given this, and the slight gap in some areas between the back and sides, I don't recommend the official 1500 without a case (on the original Incredible).
As for the battery ratings, there's a plausible explanation on why the Seido 1750 (for example) performs between the 1300 and 1500 from HTC. I'm going to do a half-assed ASCII drawing to represent it.
0%|--A--|------B------|--C--|100%
Lithium-based batteries tend to degrade over time from full or near-full charge/discharge cycles. To prevent this, you're not technically capable of fully charging a battery. There's a safety built-in. In this case, A = the first 20% of a battery, C = the last 20%, and B is the 60% in between. However, B is the "user accessible" portion of the battery. When you reach the A/B border, you're at "0%." Likewise, when you reach the B/C border, you're at "100%." Short of shorting the battery, you're not going to go over the B/C border. You can, however, cross the A/B border. All you have to do is "fully" discharge the battery, then let it sit. It will eventually lose enough residual charge to enter the "A" area. When this occurs, it becomes harder to charge the battery and will require a few cycles to get it working again.
It seems likely that when HTC states a rating, they're rating only "B" while companies like Seidio inflate their ratings by report A+B+C, even though only "B" is accessible.
Regardless, the HTC 1500 outperforms (slightly) the Sedio 1750, which outperforms the HTC 1300. The Amzer and ChiChiTec 1800s fall in there somewhere as well.
