These slots were required to band the core so that it will pass through the brazing operation. Welding doesn't seem to be very good way so, I am working on to plug-in those slots with some high temp rubber or plastic material.
How much air leaking was the one I was trying to find. I think it may not be too much. But the air static dP in those slots may be significantly smaller than the core. So, cfm of air through slots will be larger than expected.
Atlast, I used delta P = C (V/1097)^2*rho to find out delta P in inwg., where C is a loss coefficient & V vel. in ft/min.
Thanks Speco for your reply