It may be better to replace your existing transformer with an LED power supply unit.
This may help.
http://www.bltdirect.com/products.php?cat=981
I'd agree with that. The 'transformers' for most halogen lights aren't really transformers in the traditional sense, they are SMPS (Switched Mode Power Supply), and will only work properly if the connected device is within the design range of outputs. Even it works at, or beyond, the edge of these limits the transformer could be under stress and fail prematurely, possibly damaging the LEDs in the process.
I've also a personal grudge against halogen transfomers as I've discovered mine contribute to DSL interference and so reduce my broadband speed, though that may just be my brand (can't recall what it is). LED transformers would be designed for much lower loads, and so may be less inclined to generate interference.
LEDs also demand a closely regulated voltage level, and can be permanently damaged by over-voltage, which would be another reason to get something designed for the job.