You want a stock turbo to come on boost earlier?? It comes on early enough IMO.
Pretty much any "flow" modifications will make the turbo spool up earlier. So in other words making intercooler piping as short as possible and as free flowing as possible will help. Changing an exhaust manifold to a better flowing one (this is usually only for high mount manifolds because your not going to get any gains for a stock manifold for a stock turbo). Then theres options such as aftermarket camshafts and then a good tune or a nitrous setup.
Thats pretty much it. Obviously on a bigger turbo you might notice a good difference in spool up time, but for small low mounts, theres probably not a huge gain that you will notice, maybe a few hundred RPM with something like cams.
Hope that helps.
Just realised, what car are we talking about here? Or just talking turbo cars in general?