Even the most basic derailleurs shoudl shift perfectly when new - if it is adjusted properly. Generally speaking, higher end derailleurs can resist wear over time and be easier to keep adjusted.
Whatever you replace your X4 with will be perfect if you adjust it properly. Don't bother with all the 'upgrade' hype - the limiting factor is not the price of the components but your skill in adjusting it.
Edit:
Originally Posted by Atavar
I already have way too much money sunk in to this bike, so some more isn't going to make a huge difference.. lol ..
This is what is called 'throwing good money after bad.' Spend what you need to to keep the bike working or make it more suitable for you, but don't start any major upgrade or conversion projects as this is just a waste. If you want a lightweight racey high-end bike, buy a high end bike.