I've also had plenty of 90 minute connections this year from T3 to T5, plus a few car transfers, I haven't come close to missing a flight yet and usually get my coffee in CCR en route. The car transfer doesn't actually save much time, thanks to the one-way system, maybe 5 minutes or so.

If you consider 10 minutes to get from T3 arrival gate to Flight Connections (it can be a lot less), 10 minute wait for a bus (that would be a bad day), 12 minute bus journey, then the 90 minute allowance is about the same as an intra-T5 connection. If you do arrive with an hour to spare, just use the First Wing. I would go that route for 50 minute transfers too, but you do need to clear conformance for 35 minutes. I stopped running in airports a good decade ago, it's just not worth it, there is always another service and unless you're getting married or some such, most stuff can wait. If it was down to me, MCT would be reduced to 75 minutes, but BA will have made a careful calculation for the 90 minute MCT.

So I'd forget the Express Connections idea, just go with The First Wing, which is - as far as my experiences go - about the fastest way into any big airport anywhere on the planet.