The 7/8" piston M/C will give you a slightly longer stroke on your clutch actuating rod at the expense of a slight increase in required pedal effort. I run a 1" cylinder on my master cylinder. Personally I'd rather have a longer stroke and give up a little more pedal effort as a trade off. You'll be fine with the 7/8" M/C..
BTW... If you can hang with a 03/04 OEM Cobra clutch then nothing a hydraulic Cobra clutch can muster will slow ya down...
Dave