Hi
Afaik and came across dont think some rate-limiting feature in PIX btw whts the type of connecitivity u r having to u r SP is ther any Router coming in between ?or hows its getting terminated to the SP end ?
i hope if there r some routers in between (like in ur premises or in SP premises )they can always limit the in/out b/w.
i hope this may help u out..
regds