The BIG question here... is your league already in progress??? If you have already drafted, this is a big fantasy football NO-NO!
For the past five years we have been using exactly the same settings as yours, including cbs.sportsline. We basically have one mandatory sub per position and a flex SUB spot that people can use as they wish (does not get points its a bench spot). The only problem I have found is that we have a lot of trade offers but not that many trades. However the waiver is very active (even at $2 a pickup).
I don't know how active your trading is but adding another spot in your roster would take more players out of the FA pool, which could potentially create more trading. I am trying to convince my team owners for next year to add in a starting DL, LB, and DB spot in our roster (in addition to the def/st), but it doesnt look good.
Here is a little hint. If you are going to make such a drastic change (I consider adding a extra starting spot pretty big) doing it early in your league's existence will be easier than a couple years into it