YES! Autocross is generally strongly biased towards low speed grip. To that end, there are very specific setups that help.I haven't autocrossed in years but for autocross the best bang for your buck is tires and suspension. Getting the gearing right might be worthwhile but the little bit of HP you will gain from your proposed mods will not improve your times that much. If you are modifying your car you are no longer in a stock class so an improved suspension set up and sticky tires should be allowed. I don't know your proficiency level but you should also consider some driving instruction from a pro. That can really knock your times down.
I was kind of guessing that the OP wanted more acceleration for practical reasons.