Was testing some fullscreen blending and I noticed that If I had blending on with a fullscreen texture that was fully opaque alpha = 1.0, blending is faster than if it is partially transparent alpha = .4 Is there some cap limit like .99 or something that it decides the pixel doesn't need to access the pixel currently in the framebuffer to mix with and just overwrites it (straight write instead of read, mix, write) ?

What card are you using? It seems like a cheap trick some of the integrated (like Intel GMA) cards might try to do. It's an implementation-specific detail. The driver/card can do anything it wants as long as the output is correct.

Does your texture have an alpha channel? If I'm a clever driver trying to squeeze frames out of a cruddy card, and I see the material is set to alpha 1.0, and the texture has no alpha, I won't bother with alpha blending because I know what the result will be... Same if alpha is set to 0.0; I don't even have to draw! (except to depth buffer)

Can you show the code you tested with?