Mileage - I think some High Speed Trains have managed 7 million miles
London to Edinburgh and back twice a day = 1,600 miles, x 364 = 580,000 miles a year. Some of them have now been running for thirty years so 7 million miles is probably a very conservative estimate.
The original V12 Paxmans fitted to the HST sets were some of the nicest sounding engines ever built, and only now are being replaced by new 10 cylinder MTUs, which sound horible, but are much more powerful and fuel efficient.
NN
Very little extra power - TBH I prefer the VP185 for replacement lumps
I guessed at 7 m as average annual mileage is about 1/4 million
I believe the MTUs have been chipped to reduce their maximum output to just over 3,000 BHP, thereby retaining the original alternators and traction motors. (The MTU is rated at about 4,000 BHP otherwise).
I have often wondered how a Paxman would run in a car, but the load on the front axle would be a bit excessive, and you would need a periscope to see over it!

NN