If it attempts to directly modify the exe, Steam is going to throw a wobbly as exe will not match the one it will be looking for. Otherwise, you're pretty much good to go. Just be careful if it's a multiplayer game using VAC servers as if it does trigger VAC -- even if it's not a cheat -- Valve will tell you to **** off.

I think this is only for VIsta or higher, but i can't say for sure.
You could also stick it in a batch file... particularly if you want to run other programs (such as slow-down ones).
what does the /high do?
(as a side note, for anyone using laptops (or maybe other speedstep processors) you might want to set up a power profile with a constant cpu speed. i think that processors increasing or decreasing the speed to cope with load is another common cause of timing issues. If you do something along the lines of setting the min and max speed to 50% then it *might* help in some cases. )

I tried your shortcut, and Sacred started to launch, then appears to have closed.
I also tried manually setting affinity, but I was still running at 8 - 15 fps.
Guess I have some other issue...

Windows Server 2003 and later, to be precise (which is not Windows XP or earlier, if anyone was wondering). At a guess, sets high priority for the process.
If you really want to optimise performance of your single-processor games on a multi-processor system, you can shift other processes -- anything remotely CPU-intensive, at the very least -- off of CPU 0, run the game at high priority on CPU 0, and then afterwards you might want to revert the first step. That means the game has a CPU core largely to itself. I've discussed this briefly in the thread I linked to at the start of this one, and if anyone has other/better approaches (mine is a rather brute-force affair) I'd be keen to hear them.