who cares about these frames as you design the game taking this into account
Yeah, but FPS creators never do - they push as much stuff in as possible and in a FPS framerate is one of the most important things.
I think your guess of 5% to 15% performance loss is a little off. IE is bloated, slow, full of bugs and loopholes, and to have to rely upon it to run a game is ludicrous. It can't even display webpages safely or reliably. Ask it to run Doom4 and you might as well be playing via email.
It can almost instantly start depending on what you do and how you stream
How can it possibly be quicker than a game loaded on my HD, unless you are suggesting that it can stream the data down my phoneline faster than the PC can haul it from the disc?
and it's crossplatform out of the box
Really? The amount of webpages that don't work because a PC or Mac or whatever because it doesn't have the right codec installed or doesn't have Flash12.2.4 or some such rubbish or Java isn't on or you have the wrong BIOS version or it's Tuesday and not Thursday etc... is beyond belief.