I think you're missing a trick there Dave, IMO, hole coring is a specialist skill, and I charge for it. I don't see it any different to profiling or mitring.
No need for GTR30 bits IMO, they wear out far too fast. Prefer taking an extra 2 minutes per hole for vitually pennies rather than it costing a fiver a hole to cut.
In a word - YES, it's not every tiler that will do mitres, some don't even bother putting trim on. It's a skill and customers should pay extra if they want that level of finish.
Offer them the trim and tell them say for an extra £40 you will mitre round the window. If you can show them examples of your mitres they may well take the bait.