The original G29 mag spring should still be fine. Try it. Too strong of a spring will push the next round too hard against the bottom of the slide and can slow down the action, which may or may not be what you'd want. I'd be concerned with feeding reliability with a long spring.
I actually carry with the original G29 mag + PGrip, for ease of conceal and because I know it works reliably. But I have extra G20 mags very close to me...