I have indeed heard both - the SI I heard was a 15 though - still sounded incredible. The RL-p works great with 600-700W RMS as the site says, the SI is rated for 900W RMS.
Regarding paper cones - check what Velodyne (SPL1000 I think it is?), Tidal audio, Scan Speak, some of Seas' drivers I believe, Morel, etc. use. :P
With the power you have availble, you might want to check into Image Dynamics subs also - their IDQ line might be perfect for you, also a bit cheaper. I just picked up a couple of 10" IDv.3s last night to put in my girlfriend's car... two 10s sealed going in the back of a '98 beetle with trunk space left over:
