I'd argue that.300bhp and 450ft lbs is probably enough for a big diesel cruiser, so for my money I'd take a remapped 525/530d over the 535.
That said I had the same lump in my 335d and I couldn't help wondering what a remap might have done for that... 
On the 530d, Hartge tuning use to replace the intercooler with a larger one and remove the electronic limiter when doing a remap. This would increase the top speed to an alleged 170’ish mph. This puts it in line with the D5 Alpina (535d) which has a 170mph+ top speed.

I’ve had enough of these to know just how quick they can reach the 155mph limiter. They pull like a train to around 125mph, then have a very slight lag for a few mph and then pull like a train again to the speed limiter.

It’s a lovely engine/gearbox combo, and is plenty powerful enough without any mods.

If I ever become interested in speed again, this is the engine I’d go for.
