earthing kit wouldnt do THAT much. it may give u a tiny bit, but its very minimal in terms of results gained...compared to one that doesnt have it. maybe 1-3 extra kw

and firstly find out if ur car was in 3rd gear or 4th gear when doing the dyno
from what i know, 3rd tends to give a higher reading when pulled all theh way to redline than when a car is dyno tested in 4th gear all the way to redline.

if it was done in 3rd and u got that figure, then get ur A/F ratio's checked. make sure ur not running rich.

then dyno again