Two issues to consider:

Boost is usually much lower on the RR than the road due to the load not representing that seen in 4th and 5th gear. As such you have to look at what power you have made for the actual boost on the RR. For example if it was only holding 16 psi then 300bhp is a good result.

L1, L6 and L8 (except for pectel strategies) all use boost reduction strategy as ACT's rise rather than gradual ignition retardation. This means if you have passed a critical ACT condition on the RR the amal valve may have completely shut down causing a rapid reduction in boost and the type of power curve you have there.

If it holds it's boost well on the road you have no issues, and is just another problem typical of rolling roads!