If lots of extra boost pressure fails to yield any extra power then you have most probably either:

1) Reached the TIP max.
The head cant flow anymore volume due to turbine housing/manifold.

2) Hit a huge flow restriction in teh exhaust.
The turbine cant flow any more volume due to backpressure in exhaust.

P.S: Both the above will normally show unusually high EGT's.

3) Hit a flow restriction in the inlet/head
Not easy to measure in absolute terms on a speed density system. Try measuring TIP and turn teh boost up to see if it raises.

4) You have run off the compressor map and into excessive compressor heating.
Have you measured the Air temps at the inlet when Mapping?

4) Camshaft misstiming is causing a massive drop in VE.
What are teh cams dialled in at? Is it laggier than expected?

One of the above will very likely be your problem.