You can certainly do the LHR-LGW transit in 2:30 or 3:00 - I think my last transit came in between the two - and I was a naive arrival into LHR T5, not knowing where the buses were since I had expected to arrive into LGW and had been forcibly re-routed due to a cancellation. However, since I was a domestic passenger, I didn't have to clear customs or immigration - but I did have an inordinately long wait for bags.

But I agree - I don't think that 2:30 or 3:00 could reliably be made by the majority of passengers. However, maybe BA are happy to take the hit on the misconnects by being able to timetable greater connectivity between LGW and LHR flights routinely. For example, they can cut back on domestics into LHR by putting more flights into LGW and say our flight options are not materially reduced - always providing we can make those flights of course

But before I go into a full-on BA/Regions rant, I'll agree. I think around 4 hours sounds more sensible than under 3.