Heat treating does not increase a steel's ultimate strength,...
I think something got garbled there. For example for 4140:
95 ksi tensile, 60 ksi yield in the annealed contidion
257 ksi tensile, 238 ksi yield hardened and tempered to 510 HB
(source, ASM Metals Reference Book, 2nd ed)
For the OP: I'm a hobby metalworker. I have the heat treat oven, the Rockwell tester, and so on, and have heat treated knife blades, springs, and so on. I would not in my wildest dreams redo the factory heat treat on anything safety critical - guns, car suspensions, etc. It's just too easy to get it wrong (and too likely the factory already got the optimal balance). Moreover, it's hard to say what the weak link of a design actually is; if you make a 1911 barrel stronger and shoot uber hot loads you may just find that the link was the weak part. To look at it another way, what reason would a factory have to ship a gun with nonoptimal heat treatment? It doesn't cost more to do it right.