Yep Swindogs comments are pretty much right. The only thing they really check for a cert when you have adjustables is whether the springs are still captive at full stretch. They'll hoist the car up and check for movement. If there is movement, you'll either need to raise it back up, or get some keeper springs that sit on top of the coils.
Height of the exhaust off the ground doesn't matter, the lowest structural point of your car is normally the rear diff. I had my car lowered on Teins pretty much as low as it could ever go, and the diff was still 100mm off the ground so it would basically have to have no springs at all to be too low.
Once your car is certed, the height from the middle of the rim to the guard is measured. If you change this height within +/- 5% then it's still okay to pass a warrant. Any more than this then you'll need to get it re-certed. For instance, if the measured height is 300mm, then you can raise or lower it 15mm either way with no issues.
I did however get warned that the coilovers I had were very, very stiff and it could cause problems with other parts, but it wasn't a certification issue.