Nice work, but the fact that a straight port of a NES emulator already runs slowly on a 360 tells me enough about XNA already. Sure, XNA wasn't designed for this, but a 360 literally has about 3000 times the processing power that a NES has. Even an unoptimized, naive, 100% interpreted (no dynarec) implementation should easily be able to run at full speed.
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So I think we can safely rule out PSX/PS2/PSP/NGC emulation using XNA. SNES/GBA/Megadrive might just barely be possible if some really smart guy throws his skills into it. I don't count on it though...