riffraff,
My R9S has been pretty reliable with all the Speer GDHP loads, but I've had some issues with other premium brands as well as Winchester White Box FMJ (cheap stuff

).
My conclusion is that the R9 feed ramp has been "tuned" for the particular shape of the GDHP (& Win SilverTip) bullets. If you compare samples of the bullets side by side, you can see a noticeable difference in the "pointyness" of the various manufacturers' bullets.
Speer GDHPs and CCI Blazer Brass FMJs (cheap stuff) are very consistent in overall length - at least for the rounds I've measured myself.
Getting back to your questions on the 147 gr. GDHPs - which worked fine in my particular gun - I went back to my local gun shop to pick up some more. The owner is an older fellow who is also an old-time gunsmith. He touted me off the 147 gr. load. He said 124 is the classic load for 9 mm, 115 gr is okay if you are a faster-is-better type, but the 147 gr causes problems in lots of guns. He handled my R9S and was very complimentary on the workmanship & fit of the gun. I stripped it for him, and he was even more impressed, but said I'd probably find keyholing if I fired those 147s beyond 5 yards. Just not enough barrel length to get the heavy bullet stabilized.
I'd also read that the Rohrbaughs purposely slowed the rate of twist to limit torquing of the little gun in the shooter's hand. Next time I order something from Brownells I might get some cerrosafe so I can slug the barrel and measure the twist. That's not really too high on my list of things to do.
I guess the bottom line is to feed the thing the round that is stone reliable. A backup gun has to go bang every time you pull the trigger or you're better off carrying a one pound rock in your pocket. And rocks are a hell of a lot cheaper.
Steve