Feedback on astable 555 timer circuit to control MAC valve for turbo control
It looks pretty good to me.

At 4.7 ohms R4 is too low resistance, use at-least 33 ohms here
something like 1K would be a good size

R3 basically has the same effect on circuit operation that R4 does so it could be combined, however a resistor in series with D3 would create an upper limit on duty cycle so maybe you should put a resistor there instead.

To get close to a 14-78% duty cycle with a 100K pot use 
R4=18K,R3=0, and 27K in series with D3

The over-capable MOSFET will not be a problem, the larger they are the less they heat up.

R5 just wastes electricity, the 555 already has a strong internal pull down when the out pin is low.