I was able to optimize the high efficiency reactor!
1,346 HU/t, efficiency 42.07 (1,370 HU/t, efficiency 42.81 continuous)
(starting temp 6000, suspend on 6500)
I am fairly certain this still isn't the most HU a 40 eff reactor can make.
The only downside is that (most likely) the Reactor Control / Nuclear Control mod is needed, I always play with it enabled so I don't know if this reactor could be controlled by vanilla redstone.