My car is a 740T so it's a different trans than a C70. My car also was converted from a MT to AT by me a few years ago using a used trans from a '95 945T with 60K miles on it (perfect condition). The older RWD ATs were almost "indestructable" and typically last the life of the car doing 30K mile ATF (fluid) changes only.
I hated the cars MT for many reasons so out it went and no, it's not slower with the AT with high stall speed torque convertor (MVP).

The C70 (S/V70) trannies are not as tough as the older RWD ATs but IF the ATF is flushed occasionally, it should last maybe 200K miles? Depends.
I've seen many last to 140-150K miles when the customers never changed/flushed their ATF so if the fluid's changed, in theory it should last longer. They don't always however and if the fluid has never been flushed and the car has high mileage, sometimes the trans fails soon after being flushed. It's important to start flushes at lower mileage, at least I would. Some here might dissagree.