Trying to do extensive amounts of jumping in a western saddle is unfair to both you and your horse. Yes, you can manage to avoid the horn if you grab it...and you can get off their back if the stirrups are short enough, but a western saddle is still not designed to be jumped in and so it will screw with your balance and, in turn, effect your horse's balance and agility/ability.
If I was going to do any jumping in a western saddle, I wouldn't go for anything more than about a foot tall. That's just tall enough to encourage them to jump but not so tall that they really need full range of motion in their back (because of what BSMS said).
You might be interested in doing something like extreme trail riding or extreme cowboy races. There is generally something to jump in there and it can be very competitive. Plus, you wouldn't need all new tack for it. Extreme Cowboy Association