Just checking in to see how you're doing on this matter. I am in Harleysville, pretty close to you. You're welcome to take a look at my 520 to see how it compares to yours in operation or I would be happy to run over and look yours over.
The 520 is capable of a pile of heat and should have no problem meeting your needs. I would think if you had too little baseboard length, you would just bounce of the hi limit setting rather than extra long run times. Watching the boiler temp throughout the heating cycle would tell a lot about what is happening and how to improve performance.
I would recommend starting with a fixed temp on your thermostat to evaluate your system and then move on to variable settings, but only with a programmable thermostat that is capable of anticipating a temp set point/time. Otherwise, your temp "feel" will always be behind.
Do you have a manometer set up to check for draft, specifically overdraft? Maybe the heat is all going up the stack? I run mine a .020" to .025" with good results.
Anyway, let me know, I'll help however I can.