well, matk is higher in general and the increase you get to summons via SPR is higher than the MATK you get from int, 179 points of int even while riding were not that different, was actually higher with full spr than when i go 50 SPR rest int, salamion damage was also boosted. only skill that suffered was summon familiar wich already scales like a monster with MATK.
imo a summon focused sorc should invest around 2 SPR every 1 Int. if your main damage is from other source (read pyromancer), you are better going with higher int. but even then, you can get a lot of SPR from gear, windia rod gives 60 SPR alone, you can get the lvl 315 purple gear for another 50~, 110 SPR will give you A LOT of sp regen (summoning no longer locks you out of sp regen but who knows if they will keep that). as long as you dont go negative on SPR for the sake of MATK or INT, i would say you should be okay.