That is up to the individual person. The reality when it comes to this hobby is that we only see small gains when we do things. There is no one thing you can do to your engine to pull and extra 10mph out of it. You have to have a collection of things to get that 10mph... and even then, you might not get that 10 mph. A 3 mph gain from a single component swap isn't bad at all when it comes to this little engines... but that's top end and at the expense of low end torque.
Personally.. I'd stick to the MZ65 as I'd utilize the torque much more than the top end speed.