if you compare an a15 to an e15 they both would need the same work
to make the same power. An e15 cannot make 150hp easily and neither
can the A15 so relatively speaking the same is required for both.
A turbo changes the power and in most cases whether pushrod or ohc
the playing field is reduced further as the worst engines in NA setups
will work very well with a turbo. The e15 has no advantage in my eyes
if you have one of each in your shed. Id prefer the A series and beyond
that go the qg18de or ca18de for a real evolutionary upgrade.