It's not that the 55 Watts HID kit that's dimmer than 35 Watts one; it's the shift in color temperature. If you drive the 6,000 Kelvins 35 Watts metal halide bulb quite hard, let's say the 55 Watts ballast hits it with 49 Watts of electrical power, it will "red shift" to 5,000 Kelvins. Of course, and the bulb life expectancy will also be compromised too.
I'd recommend to stick with the 35 Watts HID kit, unless you want to keep the 55 Watts HID kit, then I would recommend to go with General Electric or Philips metal halide bulb (ceramic metal halide will works just fine too, however the color temperature option will be limited, from 3,000 to 4,200 Kelvins) - just go with about 50 Watts version, bi-pin version (like G6) for the 55 Watts HID ballast and it will be all good as they're of better quality compared to what you get in the HID kit. You'd have to fabricate the bi-pin socket holder to focus the bulb in the reflector. I have been wanting to try to do General Electric CMH in the spotlight, as they last ridiculously long (15,000 hours to 70% chance of survival regarding the lifetime failure mode).