It would be something to try. It should open the blow. I would keep both slides so you can have a choice.
You should really think about putting a new leadpipe on the horn. My choice would be the Pilczuk leadpipe (excellent results). Rich Ita is the guy to contact on that. His website is:
http://www.brassinstrumentworkshop.com/
I have heard people having good results with a Bach 43 leadpipe but the Pilczuk pipe is hard to beat.
I would also consider getting some heavy valve caps and experiment with them (especially on the 3'rd valve). They make a difference in projection and focus as you go into the higher range. I don't like the heavy caps on all three valves because it reduces my flexibility - just the third valve. For relatively little money you can have a really nice playing horn with some modular options to change the way it plays.
Enjoy!
Mike