The Amtrol should be plumbed to your HEATING SYTEM water, not the tankless coil, and the boiler controls should be configured to allow cool off between burns rather than maintainging temp for just the coil. The embedded coil is is an isolating heat exchanger to keep the potable & heating system water seperate, but the Amtrol has a similar, but MUCH LARGER internal coil that does the same thing, but with higher capacity. By not keeping the boiler hot just for heating water, it's standby losses fall dramatically, and the Amtrol's insulation is at least 2-3x that of the boiler, so it's losses are much lower. Whoever hooked it up to the coil was either an amateur or an idiot, and clearly didn't RTFM. He gave you the worst of all worlds- a forever-hot boiler with high standby losses, and a reduced first-hour rating that what you COULD get out of that combination of equipment.
Plumbed as per the Amtrol spec, and controlled as a "priority zone", the tank gets the full 100kbtu output of the WGO3 boiler whenever it's calling for heat, which is enough to run a shower 24/7 and then some, if you wanted to. If the boiler had 200KBTU of output, the Amtrol could take that in as well (but it doesn't put out that much, so don't sweat it.)
I think you're misunderstanding the gpm specs between the Amtrol requirement and the WGO3's coil. Amtrol wants you to pump 7gpm at as high a temp as the boiler will muster for high turbulence/good heat exchange in their heat exchanger. The 3.5gpm spec with of 100F rise on the embedded coil is the coil's limitation on how many BTUs it can deliver through the coil, when the boiler is 200F, and has nothingto do with the flow rate you're getting out of your pump. In fact, the coil's rating for 140F out with a 200F boiler temp is 175KBTU/hour, which is almost twice as fast as the burner can get the heat in(!), which means if you're drawing hot water that fast, the boiler is going to be dropping in temperature.
The head of the embedded coil will be many times that of the boiler & heating-plumbing when running the Amtrol as a heating zone. Given the modest output BTU of the boiler itself, the head of half-inch plumbing isn't likely to be a limiting issue- the BTU capacity of the heat exchanger in the Amtrol will be several times the output of the boiler. Whatever gpm the pump was able to push throught eh embedded coil, it'll likely be much higher with the comparatively low head of the boiler rather than the coil. The head of the Amtrol's heat exchanger is also pretty low compared to embedded coils.
With even 4gpm @ 100KBTU you'd be looking at a 50F delta-T on the boiler (whihc may be near it's stress-limits- look that up), with 130 F water coming back to the boiler from the Amtrol's heat exchanger you'd be delivering 180F water to the Amtrol, and get it's 170gallons per hour rating out of the system. (See: http://www.amtrol.com/pdf/MC10164.pdf ) At 6gpm you'd be treating the boiler nicer with a 33F delta-T and it would settle in at a different temperature range.
What model Taco is it? How many feet of half-inch, and how many elbows are we talking? A gazillion installatoins out there would use a Taco-007 and 3/4" pipe with an Amtrol, and deliver good performance out of a wide range of boiler outputs.
Whatever you set, up, it's good to set the Amtrol's setpoint high enough that the water returning to the boiler comes up to 140F or higher fairly quickly on a cold-firing. That protects the boiler & flue from corrosive condensation while running. On a cold start of the boiler on the Amtrol will initally be bringing the boiler up to temp and taking heat out of the Amtrol to do this. But the much higher thermal mass of the Amtrol relative to that of the boiler means it won't drop TOO drastically, but it could be as much as 5-10F. If you install an Intellicon HW+ or Beckett Heat Manager retrofit economizer on the boiler it will purge heat out of the boiler and into the at the end of a burn into the Amtrol rather than abandoning that heat in the boiler at temps 25-35F hotter than the tank. (It'll do similar smart-sensing during heating system calls during the heating season.)
It wouldn't surprise me if plumbing the Amtrol correctly and using an Intellicon you cut your annual oil consumption by 20% or more. A 100K boiler would be 3x oversized for my ~2200' (~3500', if you count basement areas) stick framed antique in Worcester, and it may be 5x oversized for a decently built 1980s house, meaing it runs closer to 70% for an as-used AFUE, compared to it's 83% AFUE label (which assumes ~1.6x oversizing to be at all accurate.)
And just GET RID of the notion of using the embedded coil- EVER! It's just leftover bad-idea from the days of cheap oil. If you have an indirect, it will (almost) ALWAYS provide higher summertime efficiency, and improve the overall wintertime efficiency. And with a heat-purging control on the boiler it'll do even better! See: http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf Pay attention to how the steel boiler (system #3) with heat purge fares even when 3x oversized. You can approach that level with retrofit controls even if 4-5x oversized.