Absolutely no idea.
BitBlt is rich enough (color maps or direct color components, depths, endianness) to make this unpredictable by simply analyzing sender chain in source code (it's too deep and convoluted).
The best way is to try it, but then one must choose a benchmark, and here again it is a difficult decision, because maybe you didn't bench a path that some other use intensively, how to guess without above analysis?

I saw those un-necessary mismatch while analyzing Graphics code...
By a principle of economy, I suggest to remove them.
Simplifying code a little bit is already a win.

BTW, did you see that scaled rgb components use 10bits while scaled alpha uses 8?

No :) because I�m totally ignorant of this part and I need time to learn and I�m too busy and too sick because I work too much.