What you gain is about 0.400" of case length over the 45-70...Yes, the 45-90 is immanently doable, basically the same comparison as a 450 Marlin/458 American to a 458 WM running the Handi pressure limit.
As to the worth?? That is something you have to decide.
The reason the 45-90 and the 458 WM touch is because they have approximately the same case capacity...as has been mentioned before, so you are at least comparing apples and apples...case capacity for the 45-90 is ~91 gr H2O and for the 458 WM is ~93 gr H20...for all intents and purposes they are identical. The only way to know WHAT the CC is for certain for the actual case in use is to measure the volume the old fashioned way, and then do the software calculations...or...MO'BETTA...go out and do the load work.
The reamer is $26 or $36 bucks plus $14 UPS prepaid return box rental fee for 3 days, it can de done by hand very easy, the cost of brass is not all that much over the cost of 45-70 brass and you gain ~14% in case capacity, a good 250-300 fs over a 45-70...not a lot in speed...BUT...enegy is calculated at the square of the velocity or directly proportional to the bullet weight. Meaning an additional 300fs adds a tad more pizzazz over just going to a heavier bullet.
At 2100fs ME is ~3425 ftlbs...
At 2400fs ME is ~4475 ftlbs...
You decide if the additional ~1000 ftlbs of slap on the hide is worth the price. I think the chumpchange additional cost is well worth the added WHOP. Just do the math.
All figures are approximates but a quick check in any reloading manual will give you similar numbers.
Just think what the additional .750(3/4") of the 45-120 will do for you in the energy department.
Take you pick of all the good reasons why do a 45-120 and forget the "small stuff". Hahahahahah
I had BP in mind when I did my 45-120, but that black stuff is hard enough to clean in my regular smokepoles...I don't even want to think about the additional work for cleaning the brass. I popped off 5 45-70 BP rounds just for fun and cleaned the bore with alcohol just like I do in my Great Plains, but I had soot in places I couldn't imagine how it got there from handling the cases...I hosed them out with brake cleaner and decided BP in cartridge cases wasn't for me.
Luck