The main problem with Rin taking part in the Tri-Wizard tournament isn't power. The problem stems from lack of variety. If you stop and think about it, wizarding magic is far more versatile than Nasuverse magecraft, even if it is weaker in terms of battle capability and other aspects. What's more, for at least the first task, the participants are limited to bringing their wand to the competition. That means Rin can't bring jewels or other weapons and must instead rely on her wizarding and other spells that do not need a foci of some sort.
I don't know about you, but I wouldn't want to put gandr up against even a potterverse dragon one on one. Even if she has something like the Kaleidostick on her, one, that would be utterly humiliating to use, two, that may not count as a wand, and three, it could reveal her as someone from another dimension or at least draw tons of attention to her style of magecraft.
That doesn't even get into the second task and how they have to handle challenges that force them to breathe underwater for an hour. I don't know about you, but I don't believe that Rin would have any aquatic spells in her arsenal in terms of magecraft, that means she'll have to rely on Potterverse magic AGAIN.
All in all, if Rin is actually having trouble with Potterverse magic, she could have something a bit bigger than a small problem. However, if she's allowed whatever she wants in the third task she'll probably dominate.
*Giant Spider jumps out*
Rin: DIE YOU STUPID THING!!!
*Spider destroyed by exploding gem*
Rin: That's it! Screw following the maze. I'm finishing this now!
*Blows a path of destruction using jewels straight to the cup.*
Rin: Now stay out of my way!