I would definitely recommend Management Servers, instead of agents. It is difficult to put a precise number on the performance increase; but because Management Servers write direct to the SCOM database, without having to use local queue cache as an agent does, then they will perform much better.
I would say that you can definitely monitor (with full functionality) using three MgtServer-Collectors, instead of the five you have now. And that will give you headroom for some future growth also.
BTW - Our next major MP update is almost ready for beta - and that will have some huge performance and scalability improvements. I'll be dropping you a line about our beta program very soon!