Give them another ring and ask them to increase the BRAS to 38M or even 39M on Clueless.
Like I say when mine was on 32M, and then changed to 39M I saw an instant jump from the capped speed. Your speedtest result looks pretty ok to me though, not drastically wrong.
Even if it is for your own piece of mind, and not having a major affect throughput in your case then at least you know everything will be set to how it's supposed to.
The interleaving issue is what bugs me though. I see a fair few requests about it from customers on various FTTC isps, and it certainly doesn't seem easy to get it turned off, even if the BT openreach modem fails. (although as I understand it, it should just take a call from an engineer to the correct people/department).