Basically depends what you want and to some degree the existing system. Microbore isn't an issue provided the pipework is clean, most issues are due to blocked pipework, being smaller bore they block more easily. No point having a Megaflow if the incoming mains supply isn't man enough to support it, you can only get out what's coming in.
Upgrade to fully pumped shouldn't be too much of an issue, will require some additional pipework, and alterations to existing system, and the addition of required valves, and electrics. Only concern I would have is if the boiler will adapt to fully pumped without an issue, unless you're planning to upgrade that at the same time.