BELKAP, is garage 5000sf total or each level???
Are you runing a mixing valve between the OWB and the inslab? or are you runnign 180f water thru slab??
Only reason I ask is I thought my dealer told me never to run the water temps for inslab over 140f or 150f, potential for excessive cracking and degridation of concrete.
Lets do some math; say you are running at 5gpm and temperature drop of 40f (max drop with out thermostatic kicking in)
BTU= GPMxdTx500 -> 5x40x500 = 100k BTU/hr -> divided by 5000sf = 20 BTU/sf which is typically what you would want for a typical radiant application
So since you can not increase the temperature drop before returning to OWB, and still need more heat for building; you may need to increase the GPM to get more heat out of OWB.
NOTE: Most people on this forum (from experience and a bit of reading) typically run in thearea of 20f across loops.