Might be tough doing full volume BIAB with only a keggle or 15.5, for that reason I'm getting a 20gal from Spikebrewing once he gets them in stock. Woooo! My calculations using the biabbrewer.info calc shows that I can do 10 gal batches with a OG of around 1.080 which will give around 19.5 gallons full mash volume.
Of course the other option with a 15 is to top-off after mashing. I prefer the all volume though and my poor 10gal AL pot is just not cutting it - I overflow on 1.052-1.054+ OG batches.