Uh, since people want to see testing on the stats, someone over on reddit tested the 15/15 int/mind vs 30 int split for summoner (15/15 being for playing both sch and summoner)
https://docs.google.com/spreadsheet/...=sharing#gid=0
His tests are there, it effectively works out to a ~3.3% difference in dps between 15 and 30, which given that the 15 gained is ~3.26% of his 442 int, lends heavily to the theory that attributes are a straight linear curve
Taking that and using it on the OP's numbers, given he's at 352 str -> that's a roughly 8.5% gain, 12.7% under berserk.
Whether that is worth the hp lost from vit is debateable, but those are the numbers.