Hi Oldsmobile_Mike,
I've got a GVP A2000-HD+8 in my A2000 and running an 18GB SCSI drive off it. I've got the GVP ROM 4.15 from Ralph Babel's site and running PFS3ds as the filesystem. You certainly don't need the Guru ROM to access more than 4GB safely on a hard drive.
The Guru ROM does have a couple of extra features over the GVP 4.15 ROM, but if you set up your HDD properly with PFS3ds and the GVP 4.15 ROM, your access speeds and ability to read more than 4GB is exactly the same as if you were using the Guru ROM.
I haven't yet played with PFS3AIO, so I'm not sure if there's an option to force the Direct SCSI option in PFS3AIO which is needed to access more than 4GB, I'm sure the docs will mention how to do it.
I've always used HDInsttools rather than HDToolbox for drives larger than 4GB and always use the quick format option as Bamiga2002 has pointed out. I've also found that it's best to keep your boot partition smaller than 1GB otherwise it doesn't always boot.
Cheers
