Joe,
The overriding reason you would want to upgrade is to have better dust collection at your machines. The Grizzly unit you have now is probably doing an adequate job getting the chips, but the fine dust that is of real concern health-wise is likely not being picked up. You have a different situation than many here by having your DC enclosed in a separate building. That helps to keep the fine dust cloud that felt bags (I'm assuming you are using those that came with the Grizzly collector?) generate away from you and the main part of your shop. As long as that continues, and you don't mind dumping the bags, then a larger machine with a higher HP rating and most importantly a larger impeller might serve you just as well as a cyclone.
However, you are not being well served by the Rockler hoses. It may be the most convenient thing to use, but the 4" flex hose is eating badly into the ability of your current blower to move air. A 4" hose has less than half the cross section of a 6" hose. The fact that it's flex hose makes that even worse. You would be wasting your money upgrading to a larger blower and staying with the 4" flex hose. It could be the Rockler system is the only thing that will work for your situation. In that case stay with what you have, but I would start wearing a dust mask if you don't now.
I have a small garage shop and everything is on wheels including my SDD cyclone. I use the shortest length of 6" flex hose I can, to hook up one machine at a time, so I understand a little bit of your situation. I went from 4" to 6" hose when I built my portable SDD cyclone. The difference in the airflow in my system was dramatic, with no change in blower HP. Bigger is always better in blowers and DC hose diameters.