Injector will not affect greatly the compression, either the rings are leaking/broken or you have valve trouble, either way at least the head is coming off, loss of compression will stop it firing however if bad enough.
The myth about greys is just complete bollocks put bluntly, a excuse for either a crap ecu or a crap mapper, mnine runs greys at 0.97% CO idle, not even 1%!!, and seriously smooth with it, so it can be done, 137ppm as well, so immensley clean with it, considering 180,000+ miles on bottom end untouched.
tabetha