Seen it plenty of times,and not just with the 7RM but other cartridges as well.....
as a general trend, and there will be exceptions all over the place ,IME Remington 7RM barrels are likely tight,throats short...this is as it should be because Rem barrels are rightfully throated for Rem ammunition.....and tight barrels might be easier to get to shoot well.....but all these little things add up and MIGHT give you velocities below the potential of the cartridge,just as you are experiencing.
Anyway what I would do and have done and seen done with friends rifles is as follows.You are stuck with tha bore and groove diameters of the barrel, but not the throat. The Remington action will easily take a longer OAL than the 30/06 length commonly used because tha action is H&H length.
I would seat a 160 Partition in a dummy case so that the base of the bullet is even with the base of the neck of the case.This OAL will fit the Rem 700 action magazine easily IIRC.Then I would give it to a good smith and have the throat let out for normal bullet/land ratio with that OAL.Opinions will vary on how much velocity increase you will get,but IME you will get some;and you will not hit the "wall" you are experiencing with manual loads as soon as you do with standard factory throats.
Some will say that the rifle will not shoot,and this is not necessarily true;like any other rifle, it will like some bullets and dislike others.
The other thing I would do is get off the 7828(which is an excellent powder for the 7RM,and I use it myself),but in this case I would go to a double base powder like R25,which has somewhat more powder energy,and may give higher vels.Some other slower powders like H1000 may give great results also.
I would not let temp sensitivity trouble my head because I would test the loads in both hot and cold weather(which is what the chronograph is for),and shoot the rifle to determine if there is a difference in POI,vel, etc,rather than just assume it is going to happen.
Hope this helps,and good luck