I cannot sacrifice any more of my trunk space. plus putting more weight in there would not be a very good idea due to lowered suspension. the rear could sag a little and I don't want that.

 

Given what you've said...same enclosure, one sub; your only option is to find another sub that handles significantly more power (and actually converts it to output, and not just heat), fits in your enclosure, and performs properly at the enclosed volume. You will also need an appropriate amplifier to maximize the subs performance. Look at the type x with a 1000 watt amp, it performs in a .5+ ft^3 sealed enclosure, should be about the same size (though deeper) as your type r, and handles 1000 watts.