In this blog post I test the limited edition Fostex FE108SS-HP fullrange driver. I purchased these drivers for my own personal use. These drivers retail for around $230 USD each.
The driver was mounted in the 1630 speaker which normally houses the FE108NS. I disconnected the lower 5" woofers for this test. 
I began testing the frequency response. The driver has an irregular response with a significant +5dB peak in the 2kHz region. There is some severe breakup occurring at 10kHz as well.
I then measured off-axis at 30 and 45 degrees. We can see that the breakup mode at 10kHz is severely limiting off-axis coverage above 10kHz. This means the driver has an extremely narrow listening window despite being a 4" fullrange.
For reference the published data is shown below. The graph is quite embellished due to the 90dB vertical scale.
Not to be petty, but my graph looks even better if I apply a 90dB vertical scale and stretch the graph horizontally...it's ruler flat! (sarcasm).
Looking at the burst decay reveals issues at 1.5kHz, 7kHz, and 14kHz.
The CSD plot does it no favors either.
Looking at distortion we don't achieve anything better than -50dB. For reference, I usually look for -65dB or better for audiophile levels of clarity.
Even if I eliminate the bass modulating tones from the signal we still have plenty of distortion in the upper frequencies.
Attempts to Integrate
I attempted to integrate the driver with the 1630's lower bass drivers but I found little success in getting a result that was acceptable.
Subjective Listening
Subjectively I found the driver to have a nasally sound character. Even after applying some EQ to bring up the highs, I found the highs sounded soft and lacking in overall detail. Removing the peak at 2kHz only helped moderately. On the positive side it has an expressive midrange quality that is good for a small 4" driver. However this expressiveness is not any better than other Fostex offerings at much less cost. Overall I am disappointed in this particular model. I guess these drivers will serve as eye candy on my shelf.