It seems that experiences can be varied on that front. Not that I doubt your problems but I got interleaving turned off without a query (I was expecting issues when I requested it). The CS rep simply asked if I had a consistently stable line (paraphrasing).
The change was complete with 48 hours as my line rate rose by the requisite amount (from 7616kbps to 8128kbps).
Then, a couple of days later, my IP profile duly responded, altering from 6.5Mbps to 7.15Mbps (as described
here). This was reflected in real world download speeds (early in the morning using multiple downloads from Apple to max out the bandwidth - thinkbroadband's speedtester ironically doesn't reflect the speeds I can get from certain sites).
This was all a couple of months or so ago.
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BT Total Broadband Option 3 (formerly 4) ADSL Max
8,128/448 (with 'FAST' line profile)
Linksys WRT160N (v1) DD-WRT v24-sp1 std