Look at the torque being produced, as that tells you a lot about the hill climbing ability of a diesel. If you get the I.D.# of the mfgr, their technical staff can tell you anything about the engine you want to know.

For instance, mine is a 230 HP cummins with a 6 speed that churns out a little over 600 ft/lbs. of tqe at 1500-1700 rpm, dropping down to 593 ft/lbs. at 1800 rpm. At the same time, the brake specific fuel consumption (BSFC = rate of fuel consumption/rate of power production) goes from 0.326 @ 1500 to 0.333 @1800 rpm where the HP is 203. Max HP isn't reached until 2300 rpm, where the BSFC has increased to 0.365. If you're really curious about the units of the BSFC, wikipedia has a good explanation but I won't go into it here. Anyway, as you suspect, as you go past your peak torque rpm, torque falls off while fuel consumption and HP increase. After maximum HP is reached, everything starts to decrease except fuel consumption.

Back to HP.... It's simply a calculated number derived from torque X RPM divided by a constant of 5250. Don't ask me where the 5250 came from, I just remember it from my old hot rodding days, and it works. So, torque is what turns the wheels, and horsepower is simply a calculated number indicating work (torque) being done at a given RPM.

My GVWR is only 23k, with a net carrying capacity (NCC) of over 3800 lbs. and rated to tow 5k. I tow a 3000 lb. Tacoma and am not nearly maxed out on GVWR or GCWR so total weight being pulled around is 24.5-25k lbs. That 230 HP number doesn't mean squat to me unless I'm pulling Wolf Creek Pass in 3rd or 4th gear at 2300 rpm and need to keep the revs up to get rid of the heat..! I cruise at around 1800 rpm (about 60 mph) where I have close to maximum torque and have economical BSFC numbers. We traveled across many big hills in AZ, CO, WY, and the Black Hills of SD this summer, and yes we did climb Wolf Creek Pass. My little mouse Cummins motor has plenty of power for all but the big hills that will slow down almost any larger rig and it averaged 9.4 mpg on the entire trip while towing. If my weight were higher, I'd want more power but would prefer to approach it with an 8.3 liter block instead of a higher HP mouse block. Just as in the 60's, "There is no substitution for cubic inches".

So, my recommendation is to focus more on the torque of these diesel stump pullers in your operating RPM range, and quit chasing the HP number. 8)