That's interesting as I've never seen above 74Mbit down.
I'm sure he means his sync doesn't go above 76Mb, because the DLM is stopping at 4dB instead of trying 3dB.
That's certainly not a Talktalk limitation (or a limitation any ISP can have) but possibly just his lines DLM history.
It's certainly possible to get above 74Mb/s throughput on FTTC though.
With fastpath on a full 80Mb sync the BT IP profile is usually 96.79% of the sync speed. That gives an IP profile of 77.43Mb.
You can get 77Mb/s throughput with that.
With Retransmission Low the IP profile is 96.69% of sync.
With either of those you should be able to manage throughput of 76mb/s.
Retransmission High gives IP profile between 91-92%.
There's been quite a few threads over the past couple years where users have been getting IP profiles around 88% but nobody ever knew the reason why.