As an experiment a while back, I replaced an N64 cart battery. I measured the voltage of the old battery before I started, and the new battery, which was the exact same part (brand and all). Once I'd finished soldering it, the new battery was actually flatter than the old one!
I mention this because you wanna be real quick when soldering directly onto a watch battery. Bigger ones like aa's, aaa's etc won't lose as much of course provided they are appropriate (no comment on the dreamcast situation)