I’m wading in with my experiences, if I may.
I’ve ran beemers in the past (don’t now) I’ve had 330D Sport Touring’s and on those I had a remaps done. They chuck out huge torque as standard anyhoo’s but I was towing sportsboats around at the time and was looking for better fuel consumption and a bit more mid range grunt to help tow. The map cost me £250 each time and it vastly improved the cars, both in umph and fuel consumption, going from ave 45mpg to 55mpg and when towing with 4 crew and kit in I still got 43 ave.
I then changed to a 530D Sport Touring and did the same thing, same engine and similar results, just a couple of mpg lower, I just needed a bit more room and I hate(d) X5’s (which are smaller inside) I then ran an M3 which had nothing done to it and got 25mpg, to say I was shocked (expected shock) was an understatement.
I’ve also ran a fleet of SMART ForTwos for work and (one for home) of the 7 I had 4 were remapped to stage 1. I did this as a bit of fun and an experiment, none of the kids who I employed knew which ones were done, but they soon found out. SMARTs in “that” town do an ave of about 45-50mpg stop start all the time, if they’re treated kindly they will do 60mpg but we’re talking kids here. With the remap they would do about 58-60 ave and pick up and gear change better and smoother. Of the cars that had been done the kids got to know which were better to drive and run and nearly all the time the standard cars got left behind, to the point of me getting them done 7mths later. The map I had done on these was a £100.00 each and I think was worth the effort. The lad came around, stuck a PC into the dash and ran a small programme and each car was done in 10mins. With the beemers it took half a days fiddling each car as they had to take the cowling off the engine and pull some wires off etc.
Warranty issues.. As the SMARTS were done by an “authorised” Merc tuner none of the warranties were compromised, in fact I was told that you couldn’t tell if a car had had it done or not. All the SMARTs were serviced by Merc and none ever came back with any advisory notice or anything, I also called our insurance agent and told him what I was doing/done and no extra charge was levied at all.. With the beemers it was different, even though the map was completed by a “beemer tuner” the local garage did know it had been done and each time I got the car serviced by them I’d get an advisory letter explaining that beemer only warranted such work which was completed by an “authorised, ie; dealer” tuner, the guy I used wasn’t but he did warranty the work he carried out. The insurance people were also interested to hear what I’d done and loaded my policy accordingly.
Of the experience I had I’d say seek a reputable tuner, make sure the cars not falling apart before you go, get insurance advice if you need to and check with your dealer to see if there are any warranty issues caused by having the work done. (all my cars were new when done and sold within 3yrs) If you pass on the vehicle I think it’s only courteous that you notify the new owner that the car has a map (unless you take it off/reset it) as the new owner will be under the impression it’s standard.
I still have the SMART and it’s performing as it should, just covered it’s 32k miles and have had no issues at all with it. One thing makes me laugh is acceleration onto motorways down the slip road, I often get a gasp when I accelerate past someone trying to overtake me… I don’t use the car much now maybe once a month (as we have a Prius) but can’t bring myself to sell it. I like it that much.