Why? The W12 is a normally aspirated fairly lazily tuned engine in the Phaeton, it's less complex than the V10 TDI with two turbos and all the ancillaries?
I'm not too clued up on how much the Conti GT W12 differs from the Phaeton W12, but I've come across more than one W12 engines with "just" 40,000ish with rather bad top end taps. Certainly bad enough to make me question if it'll last another 40,000...
Although Saying that, mechanically the Bentley W12 seems strong. Very, VERY rarely will an engine actually be replaced, but to be honest only time will tell.