power levels are cool. Despite the numer of people who will jump to the shub's defence and state they don't have any problems with it, simply put, the shub isn't designed for hardcore networking and I think if you tax it too much it just falls over.
Modem mode fixes all the issues because it relieves the shub of it's routing and switching burden and turns it into a modem. This then allows you to use a proper router to do what you need it to do (perhaps a Cisco router to go with your Cisco VPN
)