changing the pivot balls to lighter titanium ones just reduces the unspprung weight of the arms, allowing quicker change in directions and reaction of input. Be careful though with the 3 racing and the rc hobby type pivot balls on rc market. i bought these, and they aren't as strong as the mugen titanium pivot balls. i found one was bent and it affected the tracking of the car in a straight line. i ended up replacing both sides at the from with the steel ones for safety sake.
with the clutch it really depends on the track. A lot of people laughed at me when i had issues with the xray clutch at my track about 1 year ago. The thing is now alot of people at my track are choping and changing parts in the xray clutch, whilst some have changed it completely to the sirio clutch. funny thing is i have it tuned ok now, and a fellow xray racer (kickass) uses the standard clutch also to good effect. I found the best thing was the alluminium bell, it eliminated all my clutch tuning issues, and made it easier to find a wider sweet spot. i can't stress enough how good it works witha yellow shoe
i really don't think the steel bell is any good, it was the reason for me burning so many yellow shoes...it just doesn't grip the shoe well! i think it is the hard anodising treatment they have given the bell.
set it up per instructions but the trick is to take the smaller pinion of and measure the gap with a set of feelers like these
make sure that with no internal bearing installed and the outside small bearing and other elements screwed in you have a gap between the bell edge and the large nut of .90 to 1mm.
then just put the internal bearing in, and shim it until this gap is only.01mm in size, this means the internal gap between shoe and bell is .8mm and the endplay is .1mm.
clutch spring tension depends on the track. i ran 1mm tension at my track during saturday practice when it was slippery, 1mm tension on the harder xray spring seems like a good start (but i use the kyosho white flyweight)
you can also muck around with using the kyosho white flyweights (i use these) and the orion crf square spring (some people at my track use this spring. It depends on how you like the clutch. Punchy is good, but if your track is slippery maybe use the white standard clutch shoe for a bit more slip.
also, if you like a punchy clutch, but don't want too much kick in the rear end, go to a lower rear diff oil, maybe 30,000cst xray diff oil, and go to a higher front diff oil (maybe like 150,000cst xray)
anyway to sum up, i use;
-kyosho white flyweights (xray flip weights work well but when you have a large shoe to bell gap the holes tend to elongate and deform)
-xray hard spring
-aluminium clutch bell