That's very good power for a Rover. It's early days yet and I expect you'll be able to coax a fair bit more out of them over time. How does it compare with Wildcats I wonder given a similar spec for the rest of the engine.
Here, TRS claim to be pulling "600+" with their TVR with the TA factory heads with the biggest valves offered, but I wonder if that's hyping it a bit.
Just for comparison I believe V8 Developments managed 407 hp from their 5.4 demonstrator a few years back, may have been even more, using their own modified TVR heads.
The top Aussie Group N historic race series 289 (4.7) V8 Mustangs are presently making just over 500 hp with 4 barrel carb but the majority don't get near that.
There's also an Aussie MGB built and raced by Mort Fitzgerald and now owned by Richard Milligan with a 5.2 Rover motor and Rover heads extensively modified locally in Melbourne which is making 450 hp.
http://www.videoworldonline.eu/video/QA ... OL4oSyWM4k