Would I be correct in assuming that when using PP CS5 that adding an external enclosure with four hard drives connected via eSATA that the performance would be equal to adding the same four drives internally?

Is your 4-bay external enclosure running as RAID-0 or RAID-5? Does it come with an controller part? Or it's just SATA port replicator?

If you want performance, you can get an external enclosure support RAID-0 with a dedicated RAID controller card.