I am thinking of doing this change plus going up two in the back and down one in the front.I am not that interested in top speed more into stunting and playing around inthe country.But i would like to know what top speed would be with this change can anyone give me a honest answer please?

If you're stunting, there's probably no reason to drop down to a 520 chain unless it is cheaper. Most people go to a 520 because they don't know any better, but the people that need a 520 chain do it because it is lighter.

You can calculate this yourself by finding the gear ratios.
42/16=2.625
44/15=2.933
(2.933-2.625)/(2.625)=11.7% increase in numerical gear ratio(your engine will have to turn 11.7% faster
TS=(RPM)(K) where TS equals top speed, RPM equals max engine RPM, and K equals a constant based upon your gear ratios.

Since max engine RPM is constant, solve for RPM, set both equations equal to each-other. Then, because 1.117(K1)=(K2), you will find that your new top speed will be 89.5% of your former top speed.