OT: Modified Film Processing, advice please.

[GLewis4457]

Yesterday I was the pit crew photographer for a friends SCCA race car effort
(Alfa Romeo GTV-6 FWIW) at Texas World Speedway.  The day started out bright
and I loaded some Fuji Neopan SS ISO 100 into my (required Pentax content)
PZ-20 and snapped on the Sigma 28-200 lens.  By the time we arrived at the
track it was starting to cloud over and I would have liked to change over to
Ilford HP-5 at ISO 400 (midrole change would be nice).  Anyway, I finished
the day with 1 role of the Neopan and 3 roles of the HP-5 exposed at rated
ISO.  Now to the question.  The overcast day brought on some pretty flat
lighting, with some occasional sunny times.  Today I want to develop these
rolls and I am thinking I would like to  perk up the negs a bit.  My first
thought is to just develop them at a higher ISO.  I only have Ilford
Ilfosol-S available today.  The Ilford data sheets tell me that I can get ISO
800 on the HP-5 by increasing development time.  I imagine the same is true
of the Fuji but I don't have any tech details on the film yet, got it on sale
and I usually don't use Fuji B/W.  These are important pics for me and my
friend and could result in a pic credit or two in the national Alfa magazine.
 Iwould also like to give my friend a nice enlargement for his allowing me
into the pits with him.  Advice on how to procede is welcome.  Waiting and
using another developer is an option.

Jerry in Houston