Actually, the Pioneer does not support booktype for single-layer DVD+R or for DVD+RW. However, it automatically booktypes DVD+R DL to DVD-ROM whether you want it or not.
You heard correctly, in the case of any higher-speed Blu-Ray burner on USB 2.0 High-Speed. You see, USB 2.0 High-Speed cannot sustain data transfers any faster than roughly 31 MB/s. And the newer 10x-rated BD burners can exceed 31 MB/s on the outer tracks; therefore, the burn speed is automatically reduced to 6x or lower when using the USB 2.0 connection. An eSATA connection (or, for those burners which support USB 3.0, full USB 3.0 support on your computer system) is required in order to use the full rated speed of such external burners.
In any event, you do not want to use the BD burner's full maximum write speed on any of today's BD-R media. This is because there is currently no media available which can be written onto reliably at such high speeds. BD-R media is best written onto at the media's maximum rated speed or slightly below. At present the fastest-rated BD-R media generally available is rated for 6x burn speed.