I don't play in these kinds of situations often enough to warrant paying 200-800 for a pair of custom molds and drivers, but I know people who do. I have been told that the main benefit of custom molds is better (higher-fidelity) bass response and are typically more comfortable, and typically more expensive IEMs will also have more drivers for different bands in the audio spectrum.
As far as noise isolation, the Shure's are plenty good enough for listening to low-volume music while, say mowing the lawn.
Or, if the drummer is being annoying in a rehearsal I have unplugged them to great relief.
Isolation doesn't, I think, get much better-- there is a lot of conduction through other pathways than the ear canal.
After I played around with a couple different tips (these are very inexpensive) I found that my ear canals differ slightly in size ;)
Mainly I used the foam tips which isolated extremely well vs triple flange tips, but were very uncomfortable to wear for more than ~2 hours. Is that something you've noticed too?
I think that going with smaller is more comfortable, as are the foam ones. Yeah, they are uncomfortable for long periods of time, but I got used to it after a while and fiddling with the sized helped.
I am not sure how healthy it is to wear them for long periods of time.
The real difference is comfort. I can wear customs all day without experiencing any ear fatigue. Cable micro-phonics are also much quieter.
In any case custom tips aren't that expensive relative to the benefits, so it's something I'm considering.