13-1/2 to 1 compression ...on the street?

It depends on a lot of things. How radical is the cam? If its a HUGE cam with a ton of overlap, then the actual compression pressure might be manageable enough to live on pump premium with a mild octane booster. Same if it has aluminum heads with polished chambers- that will reduce octane requirement, but as deposits build up as the engine is run the octane requirements can creep back up.

But at 13.5:1 (assuming that it was computed correctly, most people tend to over-estimate the true compression when they build an engine) then it would be pretty hard to make it live on pump gas.

Retarding the timing is a band-aid, and not a good idea. It makes the exhaust valves run cherry-red hot all the time and can cause valve seat recession and burned valves.

Look up the thermal conductivity of iron vs aluminum. Aluminum carries the heat away from the inner surfaces of the combustion chamber and to the coolant MUCH faster than iron, to there are fewer hot-spots to initiate detonation.

There are a lot of REALLY shittly-engineered aluminum heads available, too. They're not in any way guaranteed to be better than well-prepped stock iron heads in terms of chamber design, but they will still have a thermal conductivity advantage.

explain to me how overlap and not IVC will effect low rpm cylinder pressure?

You obviously understand the issue. IVC timing is the key event that controls low RPM cylinder pressure. But late IVC which lowers cylinder pressure usually goes hand-in-hand with a bunch of overlap, so the slang creeps in. Lots of overlap means that the IV has to stay open even longer after the EV closes for optimum cylinder filling at high RPM, so it leads to lower pressure at low RPM.