I actually had direct experience with this on my first X5. Around 150k-ish miles the DPF clogged and needed to be replaced. The car still ran but was a total dog. You couldn't pass anyone on a two lane road because there was no get up and go at all. This was 4 years ago maybe and there weren't a ton of options for deletes and tunes except for buzzken and JR (I think that's right). Anyway; the cost to replace the DPF at the dealer I think was around $4500 or so but the cost to send it to Whitbread and delete everything, gain HP and TQ was about half of that.

Anyway; I wouldn't wait. Depending on your mileage I would save up your money over a few month period (or however long you need) and just plan to do it proactively before you hit that 150k mark since that seems to be when things go awry. Do the same with injectors too. Start buying them now when you have the spare money and have all 6 ready to go when the first one takes a dump on you around 200k miles or so. Its a long job to do one but once you're doing the first one it only takes about an hour to do all 6.