I have an Apple Rev C SCSI card in one of my IIgs systems and it is fast enough for GS/0S use. What I like about it is if I feel like just booting from a floppy to play a game I don't get SCSI errors (I just turn off the external HD and boot from floppy). SCSI is cool because you can use all kinds of removable drives and move data around between the IIgs and other Apple machines.

Even with a decent SCSI card you need a CPU accelerator to get much speed out of GS/OS (I have a transwarp gs), and some more RAM is nice as well (4mb is enough, I have 8 and use some as a RAM disk).

Can a IIgs use an Apple laser by chance? I never bothered to try it.