From a couple decades of reading about dust collection. Best site that explains the danger of fine dust is Bill Pentz. Hard to read and long. The other best source is an Australian woodworking forum in the Dust Collection section. The mod there has done extensive testing that backs up the need to capture that fine dust at source and what it takes to do it. I'll post the links and I hope that linking to another forum doesn't get me in trouble with the mods here.
http://billpentz.com/woodworking/cyclone/
http://www.woodworkforums.com/f200
You need to understand the makers of machines size the ports to decades old standards that were to remove the sawdust and shavings you could see. Not the micron size stuff that you can't normally see that hangs in the air for many hours after the work is done and is the most harmful to our respiratory systems.
Makers of DC's quote specs for just the impeller in the middle of the airstream because that is the highest flow and it is good for marketing. The real numbers are closer to half.
A salesman in a store will sell you whatever you ask for in a price range you state when you walk in. If you ask for one stating the requirements you want they will steer you to what they have and if they don't have it tell you why you don't need it, buy this instead.
When you are new and uninformed and trusting you'll make a decision based on cost, space you have, or advertised specifications and sometimes cross the fingers and hope.
Unfortunately dust collection is the most important and misunderstood part of a workshop and done properly is, for most, the most expensive thing to get right.
Don't get mad at me but as I see it you bought based on your closet, budget, and the over inflated specs that must be better than the one you had. Now you are trying to get as much as you can from it. Venting the dust outside is good. Supplementing the small ducts will help but will not deliver tornado suction, just a little dust devil. You said you want the extra for a sander but have not said if it is a disc, belt, drum or wide belt nor the size of the machine and why it needs a second or bigger line.
I've spoken up and told you the truth as I know it. You can believe some or all of it or none of it. That's entirely up to you, as is what you do with your system. I hope you figure it out.
Pete
