Changing tranny fluids to synthetic, or changing it after not chaning it for a long time can sometimes "clean" the tranny too well, causing shifting problems and eventual failure. If the fluid has been changed properly, I'd not worry too much.
If you don't have an immediate problem, the tranny will last longer with the Mobil 1, possibley significantly longer (as in 100,000 miles longer or more). Much better lubricant.
The one thing I would do is change the filter shortly after switching -- if a lot of crud does flush out, it will end up in the filter.
An MB tranny will only take about 2.5 qt without draining the torque conveter -- most of the fluid is in there, which is why draining is recommended. Certainly, if you do switch to Mobil 1, you need to drain the converter.
The 2-3 delay cold isn't the tranny, it's the computer, and there isn't a thing you can do about it. It's there to warm up the catalytic converter or something like that, but since the computer controls the shifting, you cannot change it.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!