First off the 540iA gets better gas mileage than you'd think it does. I get 17-19mpg around town, and 25-26mpg on the freeway. All with Dinan software in it. And I leave the AC on all of the time!
It should be significantly faster than your 535iM by a good 1.5 seconds to 60. My M-Sport is slightly faster to 60 than a friends '93 3.6 M5. Granted he's caught up by 100, and will outpace me to 165. But that's 540iA vs. 3.6 M5, not vs. a 535iM. Oh, and for around town driving my M-Sport is a good bit better than his M5. Makes him a bit jealous of my car.
Yes, the trannies in the 540iA's aren't cheap to fix. My original tranny crapped out at about 155k miles. The band that holds the reverse clutch pack together broke. So I bought a used tranny out of another M-Sport that was getting converted to a manual. That one has about 125k on it right now, and might need a rebuild on the valve body. That's reasonable ($450) so I'll just pull it out for a rebuilt one.
Biggest problem with the trannies is the "lifetime" fill. People don't change the fluid. And it should be changed about every 50k or so. My original tranny did last 155k without ever being changed though.
I have never worked on an M30, nor have I worked on my M60 yet. My M60 purrs beautifully all of the time. But I have had it for just a bit over a year, so I suppose I'll have to work on it at some point.
Cosmos Black/Black 1995 540iA M-Sport
BMW Individual Exclusive Edition, 1 of 65 total, 9/25/95 Build