well the q950 is already maxed with 256mb, afaik the 72pin ram is used in preference to the 30 pin, so some improvement might come there.
The 950 has a fast scsi interface, but not a wide one - but yeah it's onboard so wouldn't have to go through a slow bus. Really have to do benchmarks IMO.

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I would double check that. I"m pretty sure that the Q900/950 uses the NCR53C96 SCSI controller which is not fast. It is a vanilla 5 MB/s controller. The NCR53CF96 is the Fast SCSI controller typically found in the later PM8100 which does have a Fast SCSI controller capable of 10 MB/s theoretical. As far as I know, none of the Quadras have a Fast SCSI controller built in.

Apple did not like Fast SCSI because it shortened allowable cable lengths and caused support headaches for them. This is why you will not find a single Apple model with an external SCSI port rated for speeds faster than 5 MB/s. They even went so far as to block off external connectors on faster add-on optional SCSI cards in later models.

Interestingly, the NCR53CF96 Fast SCSI Controller is in exactly the same package as the later Apple custom MESH controller found in the early PCI Power Macs. I wonder if they're pin compatible.