That's only 20 gr. difference in bullet weight so if you're fortunate, at the velocity it is travelling, hopefully there won't be much difference. If I were anticipating a difference I would suspect it to be lower due to shorter barrel time and less barrel rise under recoil. I would also think that the load could be juggled, velocity-wise, to make it shoot to POA. I had a similar situation with a 16 X 16 over 7 X 57 Mauser drilling and by juggling powder charge the rifle was made to shoot to POA with a 160 gr. bullet instead of using the more normal 175 gr. for that era of drilling.
I'm not familiar with the 8X57 JR other than what it is, consequently I can't offer any info on what is available. If you cast bullets I would assume something is surely available in a mold of that weight. Sounds like a fun firearm!!! Good luck with it.