My opinion is that the 48 tooth should be fine for quite a while. What you should consider is that if you have a narrower range freewheel you may have more choices with the 48 that are in the range you like. There is almost a 3 cog overlap betwee the 36 and the 48 in the upper range with a 12-30, so many of the higher gears (smaller cogs) you use with the 36 are there with the 48, with usually better chainline and less wear. Try Mike Sherman's calculator to see how things lay out.

Heavier riders in my experience also tend toward lower cadence. As you lose weight you will likely find a higher cadence more efficient, so at the high end you will be very unlikely to ever outrun the 48-12. In rolling country pedalling downhill just wastes the energy you need for the next uphill anyway.