M52 turbos are not that rare, but i haven't seen one in an e38. the reason for this is: What's the point? the time and money spent to get a
durable working M52turbo is in no relation to an M62 or M73.
by the way, why won't you use a V8 or V12? M60, M62 and M73 are extremely robust engines. with
known flaws. flaws you can prevent with litte amount of time spent (regarding the milage you can get on them).
and what do you expect from a turbocharged straight-6 engine (which was never intended for turbocharging) in an 2000kg luxury limousine?
I love engine swaps/experimenting (i mean i'm trying to fit an M60 in an e30) but why "bother" about getting more power out of an M52? the only M52 swaps i know of, are in E36 or E30s, and the main reason is, because these cars were never built for an V8 and the creators want to keep the straight-6 engine.
in my opinion:
PROs: of an Engine "modification" like M52 turbo:
-if done properly you can get incredibly good results (and torque)
-you may keep the "728i" logo on the back, and annoy new porsches, many TDIs and other cars
-individal, unique and so on
CONs:
-for the same amount of money spent to get a good running durable turbo M52, you will get a S62 including gearbox, brakes, cluster etc. out of a M5 and put it in your e38, voila, 400HP 500NM stock.
-forgot the time, sweat, and anger spent in the point before
-try find errors in the system, and spare parts if something doesn't work.
fazit:
if you drive this car daily, don't change anything
if you still want to get more power, put a stock engine in,
if you want to build up a car with more power, get another e38 chassis, and and engine, and go nuts
