Ok so running two rockford p3 15's 2.5 cubic per sub still within Rockford specs of 1.86 to 3.0 cubes.  Subs are d2 wired on a RF T1500BDCP at 2ohm so subs seeing a few hundred watts over rms should I be ok with subsonic off? Amps filter is 28hz with a 12db slope . A few people said because I went with closer to the large side I should use the filter but I'm not sure I lose a lot of low end with it on . 

Absolutely no need.  The only way you're going to get enough mechanical movement to damage them in a sealed alignment is you go get to the point where they're basically IB or if you're just a tool with the power and bass knobs.  You're good with it off.  I promise.