Hawk 250 - Thinking about another sprocket

[Bill1]

I swapped out my stock 15/50 sprockets for 17/45 a while back and was very happy with the improvement. I use my bike for mostly (95%) commuting, but still want to be able to run through some loose sand and stuff and to take on some trails.
I was thinking about swapping the rear sprocket to 39t, but was afraid that I would lose any low-end power in 1st. If I did go to 39t and knew I was headed out, I could always swap the front back to 15t and be about where I am now.

Has anyone gone as tall as 17/39 and taken the Hawk offroad?

I'm in mostly flat country and only weigh about 160# so I'm thinking it might be OK on the road. I'm just worried about bogging down in 1st offroad.