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The 15mbps sync is a bit different, it's a level below which they'll refuse to install (well they'll install if you push them hard enough), but very few people have lines long enough for that to kick in, virtually none in urban and suburban areas. Since the 80mb upgrade is done by (more than) doubling downstream line speed, anyone who gets ~25mbps now should be bumped up to at least 30, which currently is already >95% of Infinity customers anyway.

There is indeed a guarantee on throughput at peak times - though Openreach only guarnatee capacity from the exchange to the cab - from the exchange to the internet it's up to your ISP. Some ISPs guarantee that throughput too, IIRC it's not identical to the "prioritisation rate" but close - 12mbps for consumer and 16mbps for business, again to double when line rate doubles.

The reason FTTC gets so close to the headline speed is because of the heavy overprovisioning (or understating) of download speeds BT use. IIRC, last I calculated the 80mbps FTTC product actually runs on a bandplan with a theoretical max download of ~157mbps, capped down to a "realistic" figure most people could actually achieve.