You don't need a Blow Off Valve as this simply release pressureised air from the intake side when the throttle plate slams shut. The BOV is opened by the vacuum created on the other side of the throttle plate. it does this simply to try and stop compressor stall/chatter.
The wastegate intercepts air destined for the turbine and diverts it once the right pressure is reached in the intake (reference line attached to the turbo housing usually). The spring that actuates the valve on the wastegate opens at a predetermined pressure, as this spring moves up and down/and varies the bypassing of the turbine, the turbine is kept spinning at the right speed to move the compressor enough to compress the right amount of air.
So A) you need a wastegate if you want to control boost.
B) The wastegate pressure reference can be over-ridden by an Electronic Boost Controller or a Manual Boost Controller (bleed valve).
C) Some race applications don't have wastegates cause they don't want to limit boost - some drag applications.
D) VNT/Variable Vane Technologies don't need a wastegate as the turbo's vanes etc acuate to do the same roll as the wastegate.
E) Wastegates are either Internal or External to the turbo housing. Internal WG's are found on small frame such as T3 turbos and ext WG's are used on anything T4 sized and upwards.
HTH
If boost pressure is tailing off over a certain RPM then the turbo isn't capable of supplying enough air at that pressure for the wastegate to need to operate. So yes a bigger turbine and compressor is needed.