I guess most of you are aware of the compatibility issue of a plextor DVD burner and most of the external enclosures in the market. Here is my experience ( all my enclosures were purchased from dealsonic.com):
Drive: PX-712A
Disc: RicohJPNR02

chipset of the box and result:

Ali chipset (e.g. Sapphire): not compatible, unable to start recording at any speed. This seems to be insufficient power due to the weak power adaptor supplied with the box;

Myson chipset: can burn but always buffer underrun even at 4x. Burning a disc took longer than 1x with IDE connection

GL811 chipset: Same as Myson

NEC chipset (e.g. Syba): Better than Myson but still buffer underrun at 8x

Prolific chipset (e.g. Plumex): Stable at 8x, however, this chipset has compatibilty issues with the USB ports on my Dell Dimention 2400's MOBO. Only a stand-alone PCI USB card 2.0 with NEC chipset can accept it. (This result may vary as I have 6 USB devices plugged into the MOBO)

Overall, it seems most USB to IDE bridge chipsets cannot handle the transfer rate of a DVD burner ( ~22MB/S at 16x). The IDE cable is not an issue here as all of them uses 80-pin cable.
However, it is mysterious that my Pioneer A07 can burn 8x even with the low-end GL811 chipset. Did plextor lock their drive? The requirement of the power supply is also critical for plextor. My recommendation: use Plextor internally if possible.

P.S. All the enclosures I bought has a common problem: their "ball-bearing" fans are actually one ball + one sleeve bearing. They last no longer than 3 months. I replace all of them with Delta two-ball bearing fans purchased from Ebay. You may simply disconnect the fan if you do not like it. But for certain enclosures with integrated PSU a fan is critical.

BTW, the PX-716a gave similar results.

I had the same problems with my PX-716A when using it in an unofficial external case; the drive couldn't reach speeds over 12X. At first I though the problem was the interface (USB 2.0 being too slow) but, when I replaced it with a real PX-716UF, the problems were gone (read; 16X was possible without a problem). I don't know why the PX-716A doesn't work well with unofficial cases but maybe it has something to do with the UDMA mode the PX-716A supports (UDMA mode 4 instead of UDMA mode 2 most other optical drives support).

Another comment on these issues...

With the 716A, it has been reported (other threads) that the Prolific PL-3507 ChipSet Cases worked A-OK up till version 1.04. From V1.04U to V1.06 DVD disc seam to burn at 16X. But a pixcellaction problems has been reported. Something about the 3507 just can keep up with the 716A.