Does the Microchip/SMSC PHYBoost technology help with Rx, or does it boost Tx only?
From the datasheet, the only reference I can see to PHYBoost is:


  PHYBoost which enables four programmable levels of USB signal drive strength in USB port transceivers


That indicates that basically it is controlling the drive strength of the transcievers. Drive strength is a property of transmit only, not receive.

More generally, from the link you posted to the technology brief, "PHYBoost" is basically appears to be just increasing the output voltage of the eye to try to compensate for signal attenuation.



For some background:

Badly designed transmission lines and connectors cause impedance mismatches and signal reflections that basically distort the "eye" of the signal being transmitted (basically changes the rise and fall times, as well as the peak voltages). If the eye is distorted too much, the receiver will no longer be able to confidently determine whether a 1 or a 0 was sent, causing an increse in the bit error rate.

There is very little you can do about this at the receiver end. In some systems you can overcome this using oversampling and filtering or equalisation techniques. However these are not supported by the part in question - as such once the signal is distorted to the point where it violates the eye requirements of the receiver, the information is already lost.

Fortunately, the distortion from reflections and frequency dispertion cauused by transmission line effects is fairly constant for a given system (that is it doesn't vary much over time). Because of this, it can be possible to improve the eye by changing the shape of the transmit signals. You basically predistort the signal in a way that once passed through the transmission line gets distorted back into the correct shape.

Some systems like PCIe have training mechanisms to determine what if any distortion (pre-emphasis, de-emphasis) of the signals can assist. Other systems have non-standard (IC specific) ways of the ability to do some distortion based on configuration settings.

On further inspection it seems that the PHYBoost isn't doing any clever predistortion, it's just increasing the differential voltage of the transmitters.