David is certainly the builder here, but let me give your stud spacing question a try.
2x6 on 24" centers conforms with code most places. Generally there is a minimum sheer wall requirement that specifies plywood sheeting with a minimum nail schedule at corners. Once you have met the minimum requirement, you have the choice to continue with plywood or OSB sheeting, or to change over to a non-structural building sheet. That options is clearly less expensive, and is the approach generally pursued by the production builders. (you know who you are)
24" centers are not a problem at all as long as your permit application specified that in the framing schedule. The only circumstance where it could make a significant difference is if the building must sustain significant loads (snow, tile roof, more than 2-stories). Hardie plank siding is a very stable product I don't belive it substitutes as a structural panel for the sheer wall. In support of what David mentions, studs can twist. This is a much greater problem if you are using "green" lumber (no stick or kiln drying) like Douglas Fir. If you choose to use a non-structural sheeting, you will have to judge the potential for twist in your framing lumber.
Using 16" rather than 24" spacing requires 33% more studs which seems like a lot, but stud framing materials are a relatively small fraction of the total cost.