I DID add extra preload to my stock 2002 springs before I changed fork springs. Yes, it will be a small improvment over stock. I also added about 20ml (cc) of oil to each leg as well. The net result of these two changes were a fork that was OK on very small bumps and OK on big hits at full stroke, BUT that knifed through the centre of the stroke way too quickly.
The net result is a fork that is slightly better than stock, but not great.
Stiffer springs really make a difference.