We have run at about 1.5 bar, with that amount the exhaust pressure still stays below the boost. We don't have any electricity to control the boost at the moment, just the throttle. Here are some boost/exh press numbers.
rpm, boost, exh press
4000, 1.44, 0.64
4500, 1.5, 0.72
5000, 1.56, 0.8
5500, 1.57, 0.89
6000, 1.55, 1.02
6500, 1.49, 1.12
7000, 1.38, 1.22
The unleaded is because of the rules. We could also use E85, but no one has really tested the local stuff and I believe it could also test our fuel supply taking it to the edge. Now we have only one Weldon pump there and 160 lb injectors (I think they should be anough for E85 too though). The lambda numbers were around 0.83, so it's still on the safe side.