you can use HSPICE on some (few) select paths to get more accurate timing results than what you get from STA.
take any project you have, pick the most critical path and export it to a spice file. simulate it. be prepared to see 1-3% difference.
you can also calibrate error margins in physical synthesis with HSPICE results that way.
SI, assuming you mean coupling/glitching effects, will be more detailed in HSPICE too. but it comes with a cost, you cannot do whole chip SI analysis in HSPICE.