While the Hub signals may be lower that other hubs. I would think once you successfully reach a repeater the hub signal level is irrelevant. So what you might look for is a repeated with good signal level.
DrHaas *** has created a thingshield replacement using a TI cc2530 Zigbee module. This module has a connector for an external antenna. I would try something along these lines before opening my C7.
***DrHaas
I have several (4) of the cc2530 Zigbee modules working in my system. They work well and have been solid. I don't know how strong their signals are relative to other devices but one of them is about 7 meters from the hub through two walls.
