Your reference to a "hard pedal" would typically suggest a booster problem. You may find that your vacuum booster is simply in need of replacement. TJ's generally have pretty good boosters, and you should not be getting a hard pedal unless you have converted to a diesel and the vacuum pump is failed... I am not familiar with any hydroboost retrofits, but I've honestly never looked, as there's generally no reason to... Replacing the booster if it's going "hard" would be the first step toward better brakes...
I would also suggest you stop listening to your mechanic. Drum brakes typically take LESS pressure to achieve the same braking force as compared to disc brakes. The disc brake TJ's basically have a prop valve that really doesn't proportion much. It is possible that Wilwood used a significantly larger piston for the rear disc conversion to make up for the lower pressure that the rear drum prop valve would supply, but I kinda doubt it... I'll leave that google search up to you, but if the piston size is similar to a disc TJ (or Explorer or ZJ, which all use the same caliper size) it's likely that your drum brake prop valve is likely crippling your new disc brake setup by reducing pressure more than necessary.
IF this is the case, you can swap in the prop valve from a disc brake ZJ, or a Rubicon TJ if you can find one. You will likely find that you don't really even need a prop valve, so removing yours in favor of an "adjustable" prop on the rear is another option. The rear discs will likely be much happier with a bit more pressure.
Finally, most of your braking is from the front anyway, so if you're not getting the braking you want, a front upgrade may be in order. There are "kits", like WSS pointed to above, but if your front brake rotors are glazed, they'll be far less functional than good pads and fresh rotors... A few of my friends swear by PowerStop drilled and slotted rotors, but I prefer to simply run a good set of semi-metallic pads for best braking power. Another option if you're running tires larger than 33" or a have a 6000lb TJ is doing the WJ brake conversion. Bigger rotors are always better...
Good luck!!
