Each engine combo can be different with regards to timing. The factory settings were just that, for the factory engine. Often the initial can be set higher and run better. You also no longer have the vacuum advance, so your initial setting wants to be higher without it. Vacuum advance adds to better fuel mileage when cruising under light load/high engine vacuum.
The key is not to have too much total timing, as that is where you destroy the engine - ie burn a piston or hammer the bearings away. Personally, 36 degrees total in my book is about the limit and I would feel safer with 34, but from what I read, the ethanol in the gas will allow for slightly more timing advance - so IF you are not hearing any "pinging" then I suspect you are OK. A stop limiter could be added to the mechanical advance in the distributor if you like your distributor's initial timing and it's curve, but find some pinging due to too high total advance.
The problem arises when you don't here the audible detonation. Outside temps & under hood temps can rise and this can create a problem that leads to detonation. Modern cars have knock sensors and can automatically adjust timing to compensate. Our older cars don't have this feature.
If your car starts well at 15 degrees initial, then that is also fine for your application. It is when you have starting problems, the engine struggles to turn over, that it may be too far advanced for the initial. So if it starts good and runs better as you stated, then leave it.
If you can lug the engine and it does not ping/detonate, then it should be OK - old school way of timing for maximum advance, right?