I've never heard the term 'high-speed' brake before, and I think what you must be talking about is running with one E70 isolated (must be rear one), which from memory restricts max speed to 100mph, which would be fine other than the brake would respond a little more slowly.
I've never heard of one of our sets running with an E70 isolated, the brake issues with our sets are generally more to do with sets struggling to build air, or bogie brakes sticking. A lot of the time, emptying / recharging distributors would probably overcome the issue but the time in drivers diagrams (12mins to mobilise an HST) is too short to start mucking around with distributors without getting very late departures, so they tend to get cancelled or do set swaps.