Cloning note

I had a chat with my colleague about why Clontech's vectors produce much weaker protein expression than other CMV-based vectors (such as pCI, pcDNA3, pRK5) do, even though they use the same CMV promoter ? It seems like following features of the clontech vectors make them less efficient:
-Clontech vectors use early SV40 polyA signal instead of late SV40 signal. The late SV40 is ~5 times stronger than the early-SV40 (According to Promega note).
-Clontech vectors do not have introns between the promoter and the SV40 polyA signal. It is widely believed the presence of introns is required for optimized protein expression.