Of course changing the bars for another style has a massive impact: that's geometry for you.

But without changing geometry, replacing your OEM wheels with a lightweight tubular wheelset will give the most noticeable difference, then maybe tires and chain. Brakes that work in all conditions help too: even just pad material.

IMHO, the crank is fairly insensitive unless you change ratios. It's hard to find classic cranks in a compact size, though... and who ever though a 52-49 combo was useful obviously does not live in the mountains.

Saddle? who sits down to ride hills?