the 1150s is a H1 bulb, 12v55w, so i guess the RT is similar, I'd look on carbulbsdirect or similar on t'interweb and you'll find some 'brighter' than standard bulbs, if you go higher wattage may get additional heat/blown fuses. Or go for a hid setup, checkout Les Wassal of this parish.
CheersJohn B
HID is going to give you the brightest - but you may run into dazzle problems on dipped beam unless it's a projector style lens (like the GS dipped)...