Connecting two USB DP/DM
My reading of the STM32F205xx/207xx datasheet, section 5.3.16 "I/O port characteristics", Table 41. "I/O static characteristics" is an I/O pin has a typical Cio (I/O pin capacitance) of 5pF. 

All I can find in the usb_2.pf spec "7.1.6.2 High-speed Input Characteristics", subsection "Lumped capacitance guideline for the transceiver component" is:
"Capacitance to Ground on each line: CHSLOAD ≤ 10 pF"

So I interpret that to mean the STM32F USB FS peripheral pins, when set to GP I/O high impedance, might account for 1/2 the capacitance guideline limit.

There are also a bunch of tighter timing specs for USB HS. Have you got equipment able to measure those accurately enough, or can you hire test equipment?

Do you have any other loading on the USB D+/D- signals, for example ESD protection which might also contribute signal loading?