Given a 9.3x74r in a strong single shot and 26" barrel, what can one boost performance to?

9,3x74R doesn't really need a performance increase ... it and the 9,3x62 both kill well beyond what can be expected from the velocity and energy numbers.

Just use a good bullet (like a 286 gr Nosler partition) at any velocity above 2200 fps and go hunt with it. Just load to the sights if the rifle sights were adjusted to a specific load. Field performance will be fine.

My experience has been with pigs and whitetails with the 9,3x74R and with pigs, whitetail, Impala, Warthog, Blue Wildebeast, Kudu, and Zebra with the 9,3x62. Both calibers hit like a mack truck and deliver phenomenal penetration.

Brass will limit you a bit. The RWS brass is much thicker through the web than Norma. I use both to make the .338x74Keith, a wildcat round in my Ruger #1. It's a necked down & blown out 9.3x74R. I run 225gr bullets @ 2800fps & the Norma brass gives head seps. after 5-6 firings. The RWS had yet to show any signs after 8. So I would bet you could get another 100fps or so in a strong receiver like the Ruger #1.