So, if I follow what you're saying, for a 7-gallon coil to sustain that output, the boiler water needs to absorb something like:
7 gpm x 8.33 lb/gal x 60 min/hr x 80 deg rise [assumes 45 deg in, 125 deg out] x 1 btu/(deg rise x lb) = 279,888 btu/hr.
That's higher than the gross output of an S-20 (let alone the part that makes it into the boiler water), so a 520 generally wouldn't be able to sustain 7 gpm dhw output, while the bigger EFM's probably could. A 7 gpm coil in a Highboy might initially be able to put out more dhw than would a 7 gpm coil in a regular 520 due to the btu's in the Highboy's higher water volume, but in the longer term it, too would have difficulty keeping up.
Is this a fair summary?