Bullhorn handlebars or extension grips?
I'd give the bar ends a try. They give you a hand position or two more depending on model, and they're relatively inexpensive. Plus, I've never had slippage with my Ergon GP3's -- the clamp mechanism is excellent.  

With new bars, you have to make sure that the clamp diameter of the stem matches the bar's clamp diameter, and the grip area diameter matches the shifter + brake clamps. There are some that 23.8mm (mostly used for road bars) and others with 22.2 mm (mostly used for flat bars) for the grip area. 

As for bullhorns being bare metal, you wrap them in bar tape, just like drop bars for the road. 

I'd also play with other adjustments on the bike, such as saddle position, bar height and stuff (an adjustable stem or quill stem if you have one makes this easy). You may be riding in a bad position or something which is causing the discomfort.