I agree. As a quick check, see if bumping up the pressure 2-3 psi will help. If it does, end of story.
If not....
You are right about the primary/secondary, and yes, that would be a substantial amount of work. Figure 16 on page 17 of the manual shows the boiler piped with closely spaced tees and circulators on the heating zones.
However, I re-read the manual and changed some of my thinking. Here's the manual I read:
http://www.raypak.com/uploads/2100.50AA.pdf Your system should be piped like Figure 15 on page 17.
That shows a 'pumping away' setup, because the expansion tank is attached to the system on the inlet side of the circulator. They are doing this in part to ensure positive pressure through the boiler. That's fine. Nice simple setup they have there.
Table J on page 16 says the total system pressure drop should be <8.5 ft of head. I would venture a guess that in order to flow at an adequate rate to get the water through all the baseboard takeoff sections, the head of this system is more than 8.5 ft. In theory, the B&G-100 has a shut-off head of 8 ft. i.e., if there's more than 8 ft of head resistance, it won't flow at all. But most manufacturers understate pump performance so you probably have what you observe: some flow, but not enough.
Assuming your boiler is piped per manufacturer's instructions, then I think it would be reasonable to try one of the 3-speed circulators and see which speed works best. You should be right around what they call for in Table J for flow rate and head, i.e., it's not like you are trying to double or triple the flow rate through the boiler.
Adding a temperature gauge on either side of the boiler would also allow you to see what the temp drop is during the heating season, and make sure you have adequate flow through the boiler.
BUT, this is definitely where I get nervous about being a total non-expert giving advice over the internet. You should check with a Raypak rep to make sure what you do is acceptable to them.
If you have calcium-rich water, you might consider filtering the water first. Once filled and purged, the system shouldn't require any more water unless you drain for servicing.