I'm considering buying a Geddy Lee Jazz but I'm wondering if my projected string setup would result in neck problems with the Geddy? Or just be too large at the nut regardless. Currently I play an MIA P-Bass w/.125's tuned to C. Basically the BEAD strings from a 5-string pack. They might even be .135's, but I'm pretty sure they're .125's. In any case, they work really well on the P-Bass, but can the same be done on the Geddy Lee? If not the Geddy, what about on a regular Fender Jazz? Any insight is appreciated. Thanks!
There's probably less tension on the neck with the .125 tuned to C than the regular G string would have had.
I think the neck should be fine. As long as you are tuning the strings down, the larger gauge won't really result in much/any more tension than normal strings tuned to EADG. The nut I'm not so sure about. Those would be awfully large strings, so you might need the nut replaced.
As others have written, lower tunings with thicker strings do not increase tension. A B string tuned to B has about the same tension as an E string tuned to E. You may run into trouble with the bridge, the tuners, or the nut.