I was thinking that the Ashspec core is on the thick side. The thicker the core, the less efficent it is from a heat exchagne standpoint. Most of a cores heat exchange takes place in the first 1.5" of the cores thickness. Very little takes place past that. So adding thickness increses volume and adds lag.
However the thicker part also acts like a heatsink so it could work bettwer intialy. Maybe its a wash as many cores are 3.5" thick or so.
Because of this I was going to use Stillen stuff modified with bigger inlets and outlets.
All of this is my opinion and I don't have data to support my opinion.