What you set your primary connection bandwidth to doesn't really matter. Setting it to higher might give you and the secondaries connected to you a little bit better search results, as your searches are spread through more routes, but setting it to the lowest bandwidth setting isn't bad for the network in any way. Setting it to the lowest setting should give good search results anyway.
The only thing you need to make sure of when you run a primary connection is that you use the patch from
www.winmxgroup.com . Else you will let fake file flooders connect to the network, and all your secondary slots will be used for flooding instead of supporting secondary users on the network, and then cause more damage than good.
I'm happy to see that you want to tweak your MX to support the WPN in the best way possible instead of only thinking of yourself. That's just the sort of users we need on the network. Happy MXing