Trans man, you will probably not pick up enough ET to be measured at the track by swapping from a posi unit to a spool as long as your tires are within 1/4 to 3/8 inch of each other on the circumference
I didn't see any gains at the track by swapping to a spool on my old M.W stocker with the 8 3/4 third member, the spool was a "lightweight" racing spool also
The special lightweight racing 30 spline spools are heavier than the same brand and model of the 35 spline spools are also, maybe do both changes at the same time to get better, stronger axles also