I use PVC cages, but I've noticed with plastic that the heat doesn't get through so well. I think it is important to listen to your thermometers, rather than just your themrostats in this situation, simply because of how much heat actually gets threw the plastic. For example, to get my ball python's hot spot to 90F I have my thermostat set to 106F. I was very worried, considering my thermostat only allows to be set to 108F. However, it has been working well like this for several weeks when I set it up.

Is there a way to cool down the room temp a couple of degrees, giving more variation in temperature? Even 82F to 90F might be better than 86F to 90F, you know?
If you can aim for a 10F difference I believe that is optimal.

I don't think you need an 11" tape, but I don't know too much about racks apart from what I've read on here. I'd just try to lower the ambient room temp a few degrees, and up the hot spot until it is about 90F inside the tub on the hotspot, checking with two different thermometers (if possible).

Best of luck!