I've a slope that goes from ~15° to 20°-25°. However, it's a short run. Now, to get the best cutting, I've got to back down to a tree line and drive up in 1st or 2nd (low range). All of this is done in low range (tractor is a JD 670 with a 60" rear mower). There is a FEL on the tractor...keeps the front end down. That, and slow speeds seem to keep it pretty safe. However, mowing this area takes longer then it did with my old 48" Wheelhorse (12HP)
I've noticed quite a few farms around here (south central PA) where farmers go across slopes (tilling, mowing, etc.) I wouldn't even think about. Reckon most of these tractors can take more tilt then we can.