Re: SPD, but only for (x)?
If you realy want extra speed for this you could buy it as a duplicate with various limitaions to simulate extra speed only with firearms.
or
Buy extra speed as you describe but always buy what you have already so your phases don't change you just add to them.
e.g. if you have two speed buy another two speed only to use guns (-0.5)
On phase twelve you would go normally.
On 3 you could only use a firearm. shoot, reload, whatever.
On 6 you go again and can dodge, half move and shoot, whatever you like.
On phase 9 you can shoot.
If you change from 3 speed to 5 speed for firearms you'll be in trouble.
EDIT: I just read your edit so I would suggest 2 speed for him.
Then buy +2 speed only to use firearms, -0.5. 13 points.