With a foot of brick, 3/4" of air and plaster/lath you're currently looking at about R2.5, best case.
In a NJ climate the 3/4" cavity between the brick & lath can usually be insulated with non-expanding injection foam without causing other issues. With a 3/4" cavity it would add ~R3 to the average wall R by filling those cavities with non-expanding injection foam, more than doubling the effective R value of the wall, cutting the wall losses in half. (See:
TriPolymer,
Corefill500, there are others, google "non-expanding injection foam".) This is enough to make both a fuel-use difference and a comfort difference at the temperature extremes. It's not cheap, but it'll be worth it on comfort grounds alone, and still cost effective on reduction of heating & cooling bills.
Do NOT substitute expanding polyurethane foam pours for this appication- the risk of bowing or even blowing out the horse hair plaster is high, especially with the rusting 100+ year old nails holding the furring to the brick. Injection foam is fairly low pressure during installation, much lower than the typical pressures reached by 2-lb polyurethane pours.
Non-expanding injection foam will also reduce the air infiltration. Once you're at ~R5 walls and have double-pane windows (or tight fitting storms) air-infiltration begins to loom large in the heat load numbers. Not all air leaks are the same- concentrate on sealing any penetrations into the attic first, then any leaks into the basement to reduce stack effect drives. When you've run out of places to seal, it may be worth hiring an insulation company specializing in air sealing to blower-door test & rectify the other 3 square feet of leak that you didn't find.
If you have reasonably tight single pane windows, adding
low-E storm windows can be both cheaper & higher efficiency than low-end replacement windows.
Before you modify the hot water loop for higher flow, set the boiler's high limit pretty high (say 210F-220F), switch the zone controller to run the indirect on priority, and set the indirect to something low, like 115F-120F. Then back-to-back showers or something to deplete tank temperature to where it fires (or run the shower until the output is tepid,) then crank the aquastat back up to 150F and observe the boiler's operation during the recovery ramp.
If the boiler temp runs up to the high-limit kicking off the burner before the indirect reaches it's setpoint, then re-fires a few minutes later, improving the flow will improve the recovery rate and overall hot water performance.
But if the tank always reaches the setpoint without the boiler reaching the high limit, improving the flow won't reduce the recovery period at all.
Improving the flow may still be worth doing, since it reduces the delta-T on the boiler: A 20-30F delta-T doesn't put much mechanical stress the boiler but over 50F is probably beyond the operating spec for that boiler (I'd have to look it up to be sure but most cast iron boilers have a 50F limit on delta-T). Running them beyond spec reduces the service life of the boiler by warping the heat exchanger plates and/or stretching the through-bolts, causing it to leak when cold (or after awhile, even when hot.)
If you have an infra-red thermometer you can measure the delta-T directly by putting a spot of high-temp spray paint at both the boiler output and the return input (to equalize the IR-emissivities on those points), then measuring & subtracting the temperatures for the delta.
Lower pumping head also reduces the power used by the pump, but that's probably the least of your issues.