Hello,
as you suggested, I switched on pfSense 2.2 RC on my machine. I tried to compile the driver under VM with FreeBSD 10.1, it compiled well but the kldload didn't work, so I took the one provided by arrmo, and this time it works : the alx0 have a dhcp lease when plugged to a SFR Box. Now I will perform tests to verify tu upload and download ratio ;)
EDIT : I configured my network as I planned (Captive portal + Freeradius + external MySQL database), and the debit, when I'm the only client, is almost the same as a connection not passing through pfSense. I built this for a LAN party in March, so i'll see in a few month if the custom driver bear the load ;)
Anyway, thx for your help and advices !