My tests didn't agree with the 300% jump cap. Did some dummy testing with the following setup and results.

Test 1:

No weapons, 325% jump mod. (250% innate, 50% stmr, 25% SSP wind) Basic jump skill used. All hits came in just under 200k, none above.

Test 2:

Same equip as test 1 but with Dragoon gauntlet added for another 100% jump. All jump hits came in just over 200k, none below.

Test 1 and 2 should have been in the same damage range if there was a 300% jump cap. Only thing I can think of that may throw off the results would be if her passive and stmr require a speat to be equipped. That doesn't match up with how they are worded but I've seen bigger bugs than some inconsistent wording.