a hole on the housing. Are you referring to the little metal nipple piece that is missing on the Billion hi-flo thermostat? That has nothing to do with flow, that part of the housing sits on a flange when installed. The difference is in the valve diaphram, if you look at the pic, the hi-flo thermostat has a larger valve diaphram. The Nismo thermostat valve diaphram is like the thermostat on the left...
And for the record, Billion also offers a regular low-temp thermostat that is practically identical to the Nismo thermostat. The Billion hi-flo thermostat is JPY9800, the Nismo thermostat is JPY7000.