Here's a quick way to see whatthe loft of an iron would be. Turn the club over so that the cavity is lying on the ground, turn the butt end towards the target and then stand on the face. The butt end will lift up and that's the loft you should ge from the iron.
It's an old trick from the days I played on tour!
Typically a 7-iron should just loft up, peak and then drop down back to the ground. Only realy hard hitters ofthe golf ball generate enough initial velocity for the golf ball to fly low before it starts to gain elevation.
Na,I wouldn't say the swing plane has anything to with the lack of distance. Swing plane affects a lot of things but not distance really. I'd say you're losing power by:
1. an early release
2. not enough club head speed
3. too much loft. Check the loft of your 7-iron. Often they can be way out from the standard spec. I tested a 6-iron from a cheap-ish set and it was 3 degrees weak (more loft) that it should have.
This info doesn't solve your problem per se but I hope it'll give you some ideas. If you need more help, post again.