Expected performance of e-SATA port multiplication?

Yes. Poor performance is to be expected.

The ATA protocol was never intended to carry data to more than one drive (it was actually based on the ISA Bus, extending the communication channel to a storage drive). The IDE interface cheated this limitation by carrying data for "Master" and "Slave" devices. SATA has no equivalent mechanism however, as it was intended for only a single device per cable connection.

eSATA Port Multipliers function as an ATA target that can select multiple physical media (so only one physical device can be addressed at a time, and there is a slight latency in switching to a different device). You could compare this to old Optical Libraries.

This is radically different from SAS which was built with Expanders and initiator/target routing built-in (SAS 2.0+ functions somewhat like a switched Ethernet network). SAS is also able to bond multiple lines into trunks allowing simultaneous access to multiple devices across the multiple trunk lines.

Update:
Apparently there are newer controllers that support FIS, which allows multiple drives to be "active" at the same time. This should allow performance much higher than described above. Apparently at this time only one manufacturer makes these chips. I would still highly recommend starting away from SATA in server environments. If you must use SATA drives consider still using SAS controllers and Expanders.

Are you sure you tried controller and port multiplier supporting FIS (Frame Information Structure) port multiplying?

AFAIK only Silicon Image controllers and multipliers (SiI3726) support this, but the performance is similar to SAS of same speed (80-90% link saturation).