How much .......?
The recommended speeds of the Flight Manual are x1.3 the stall for any given weight. This safety factor already allows for gusty conditions. To add yet more speed creates the problem of the excess speed having to be lost at some point. Losing excess speed and fighting turbulence, during the hold off, close to the ground at 'minimum control' has never appealed to me.
Flap changes the stability of the aircraft for various reasons and therefore all three controls can become less effective, in turbulent conditions you want maximum effect. Additionally, although the stall speed is normally decreased by flap the additional drag leaves less excess thrust available over drag to go around or to resist sink as required. Mimimum flap, having considered the runway length available, will alow maximum stability and greatest power/thrust over drag.
80kts approach in a PA28 seems very high to me yet alone adding any more.