Get out your MacBeth chart and shoot a few frames on it with 1, 2, 3, 4 stops under ASA 100. Hopefully you are processing the film yourself, so you can develop some tests to see just how far off you will be when you process your good film.
I have pushed Provia 400 quite successfully, albeit years back, for frames taken of astronomical sources (comets, etc.). Because I use scratch-mix chemistry I found that I could vary the Metol-HQ-Phenidone ratio to yield higher contrast and better low-light level results. So I suggest you process some test strips using the 8-minute first developer and then the 11 or 12 minute time. You are not likely to benefit by trying to go farther than 12 minutes, but you might well benefit by modifying the first developer formula.
Be sure to post your results!