If there's any problem,Bill, I think it's going to be with the controller rather than the towel rail element.
I've had a look at the rails that Dimplex say are compatible. One is a BR400 (400 watts). So, according to TheChair's post, the controller is able to control 400 watts.
Reducing the voltage just reduces the current load, therefore lowering the temperature. Unless there is some other electronic circuitry with your element, it should be fine.
I've had experience of extractor fan speed controllers in pubs. Much the same principle. Fans seem to go on and on. It's the controllers that keep failing.
I've been reading reviews on Amazon for a similar product to the Dimplex controller (similar price). They work fine... for a while.
I'll bet you that in the case of a warranty claim, Dimplex would insist that it should have been used with one of their products.
I'm inclined to go with TheChair, and give it a go. I can't see any safety issue (assuming your electrical circuits are kosher.)
If it were me, I would leave it alone, and fit a 150 watt rail alongside this one, or even mounted above it.
You can then choose which one to use according to the time of year.