I saw your post on BE's is getting the response I would expect there, so I'll give you my take here:
It's absolutely possible to build a kick ass Limited gun from a stock Witness; if you're like me, it's less about saving money, and more about the project (maybe there is a little satisfaction to shooting a sub $500 gun which performs side by side with $4,000 2011s!). I actually did something similar when I bought a used 45/22, sold both the uppers and bought a 10mm longslide with the factory adjustable rear sight:
I used the hammer and trigger from a CZ75 Tactical Sport to convert to SAO (both available direct from CZ for under $50 total) along with $20 worth of springs and some polishing which resulted in a crisp #2 trigger.
If you want an adjustable rear sight and FO front, I suggest you sell the standard length upper and buy a longslide conversion like I did because the longslide is milled to accept the LPA nice and low in the hand, whereas if you put an LPA in a standard slide, it will stick up like this:
