The base wire can be thin - even if you are using the transistor as a saturated switch, Ib wont be greater than Ic/10, and if you are using it in its linear region, Ib=Ic/hFE
However the emitter wire has to carry Ic + Ib. For high currents, you need wire with really thin tough insulation, but at only 8A, 18AWG (1mm dia) wire would be fine as long as its insulation is rated for a temperature of 35 deg C above the max possible heatsink temperature. That may be problematic for heatshrink, and you may need to use enamelled magnet wire so the heatshrink isn't the only insulation layer. If you can get stranded high temperature wire with an o.d. of 2.5mm or less and 0.8mm2 or better CSA, use it.