Hitting MBT doesn't cap the performance benefit of ethanol. Better knock resistance can leave room for more boost (safely), so don't think of MBT as a hard tuning line which you should never go over. In the MK6 world, where cars hit MBT on 93, we still see lots of HP gains on E30 despite hitting MBT (mainly from boost increase). Timing isn't always everything.
Short story: I do think there is still a benefit of pro-tuning on stock turbo.