GT MTB - New tires - same size front/rear or a mix? (based on my needs below)
Looks like a good tire for both uses. The speed king would be faster on road, but not as good off road.   If your off road is tending towards non-technical and dry hardpack, consider speed king. Size down not matter - narrow for more road/hard pack, wider for more technical off road conditions. 

Consider something like speedking rear and raceking front. With more weight (70%) on the rear, the rolling resistance is relatively more important on there ear. The race king ont he front will provide more predictable traction for steering and control when off road. 

One thing to watch is sealed/paved roads, especially when wet.  As you lean into a corner, you transition onto the nobbly shoulder and loose traction when you most need it. Something like the speed king can catch an unwary rider out. 

Ideally you have two bikes, second best is two sets of rims, one for off road and other for on road.