Hi Hill,
Can't really answer your question as I've only had my
B7610 a couple of days. So far, nothing has bogged it down, but I may not have pushed it as hard as you did the 7510.
Have only mowed a hour or two with it since delivery using a 60" MMM. My lawns were grown up pretty good due to recent rains so it did work the tractor, but the only indication of tough going was a change in the tone of the hydro whine....RPM never varied. All my mowing was done in Low Range.
I was holding engine RPM at around 2000 +-100RPM and getting all the groundspeed I wanted in Low Range. Since mower operation pretty much dictates engine RPM, I question why you would want to run in High range if Low gave you the speed you needed. If the sounds emanating from the HST are any indication (and I'm a gearhead who's admittedly new to HSTs), running in low places less strain on the drivetrain. I figure that's probably because you're making use of the mechanical advantage provided by those uncomplaining gears.
As to the 7510/7610 question, I'd get the 7610 unless your dealer is offering some special deal on the smaller tractor. I believe in getting the most HP available in any given frame size. With the 7510 there's a greater potential for future regrets, especially if you'll be working on hills. The engine in the 7610 is about 7 cu. in. bigger than in the 7510, so the power increase comes with a torque increase...they didn't just play with the injection pump output to get better numbers.
I really like the 7610 and am surprised at how maneuverable it is. Mowing seems effortless compared to what I was used to. My only complaints are that the mower is a bit fussy to get adjusted properly and that the quarter inching 3pt. hitch control is Mickey Mouse compared to position control...but I can make it work. All things considered, it seems an excellent machine...I'm embarassed to own anything this nice.
FWIW
Bob
'Scuse me, I gotta go mow /forums/images/graemlins/grin.gif