If you've got a 26" tube then you might get near that 3100fps mark with 100gr bullets. Not sure about 3200fps...maybe. That's not a common length tube for the Roberts but it should help you out with the extra fps.
Temps in the 40s shouldn't be a problem for double-base (typically spherical) powders like WW760/H414.
I mostly noticed the velocity variation getting very apparent down around the single digits and low teens.
I had another thought on this at work last night too. Have you considered trying Hodgdon's new Hybrid 100V powder? It's right around the same burn rate as the two 4350s...which is pretty darn close to WW760/H414...but is posting higher velocities for the reported pressures. They've got data online for that powder in the Roberts.
I'm thinking about trying it in the .25-06 and 6.5x55 but I haven't found it on the shelf locally.
I agree, there isn't very much difference between the .243 and the Roberts when it comes to game performance except for the ability to bump up to 115-120gr if you choose.
As for proving the .257 bore is better than a 6mm? I suppose if the deer notices the difference he might tell ya, but I doubt it.
I know they both work just fine.
You sound like me...I enjoy having those "oddball" cartridges that you just can't buy ammo for at Wally-World.
Be sure to post up your results too, good or bad. I'd be interested in the range report on your load.