It is not a limit of sfz, it is the way SD3 was written. Editing an sfz file is pretty easy and a handy skill. Many synths by CW and others use the format.
A second copy of SD3 should not that big a load. I use to run two instances on my old P4 machine.
If you want to try other options, you could try loading the sfz into DimPro. If the samples referenced in the sfz are wav or ogg you could try the sfz player. The sfz player does not handle flac files.