Rarely needed. The Speed P8 cranks are pretty good, so they will be expensive to replace unless you go with a different crank. That said, if you have knee pain, have very short legs, or have pedal strike on turns, it might be worth it.
I use 165mm cranks only because I had knee problems. (I tend not to have pedal strike problems b/c I used to race and am used to keeping the pedal up on turns.) My partner uses them b/c she is smaller. We both use Shimano 105 cranks, which we found for around $20-40 on Nashbar. That said, you have to make sure that you get the chainline right. Look up the specs for the Speed P8 and try to match the BB axle length (won't be exactly the same, but very close). Alternatively, you can look on Sheldon Brown's site for calculating the chainline and try to match it more exactly with spacers.
Almost all stock bikes come with 170mm or 175mm cranks. O/w you have to do a custom order.