There are a lot of mods to that car, and some people, will be put off as it takes the car some way from standard so if they want to maintain a warranty might be tricky.
But of course there are many that will view the mods as a positive and then it will be a very attractive proposition at that price with those mods that people may have been thinking of adding themselves after buying a car.
I think with the market its a case of horses for courses! The Z4M was made in relatively low numbers and really is an enthusiasts car as we all know being quite hard edged.
So IMO good, well maintained cars should always sell well regardless of the mods, provided they have been specified and fitted correctly of course. It just might be that a heavily modified car will appeal to a certain enthusiast sub-group in the market and that can be a good thing as that group recognises that cars of that type are even rarer than the standard cars.
In terms of the market, it also creates a nice dual category for Z4M's, modified and unmodified, which considering the numbers means that within each category there are even less cars available than the total number!