Start with 3 Dram equivalent or heavier loads.
I would disregard any malfunctions that happen in the first 50 rounds as break in.
Have a blast!
EDIT,
Walmart bulk Federal Field & Target 100 3 Dram 7.5 shells my not function as well as Federal "Top Gun" 3 Dram 7.5 shells. I get the "Top Gun"...
The Fiocchi 1150 fps slugs fire fine in my SPX and FN SLP 24" Comp
The Fiocchi 1150 fps 00 buckshot is very weak in my SPX and FN SLP
The Herters 1200 fps 00 buckshot runs just fine in both
Just curious, have you ever fired any Fiocchi reduced recoil slugs in your SPX?
I've never shot their 00 Buckshot, but mine feeds the reduced recoil slugs ok. it's a 2015 SPX.