The 2008 1800 SR3 4.3L came in a 190HP and 220 HP configuration. The 220HP version came in a MPI version, while I belive the 190HP is Carburated. The gear ratio of the outdrive is going to impact this as well. I beleive your model has an Alpha I outdrive which came in 1.62; 1.81; & 2.0 gear ratio's. You'll need this along with the total weight of the boat and gear and people which are most frequently on the boat. Elevation above seas level will impact this number as well.
I would think that you would need to go down in pitch to increase your hole shot. Where are you at Max RPM's? Are you within the recommended range for the motor? If you can't reach the max RPM's, then a smaller pitched prop will increase max RPM's. You don't want to be able to exceed your MAx RPM's if you can avoid it, as this would indicate you're UNDER propped. Not being able to reach Max RPM's would typically indicate you're overpropped.
In your case, you'd be changing from a 3 blade to a 5 blade which is going to add an additional level of complexity. I would take the time to speak with Mercury about this.
Here are some resource links which may be helpful:
http://sites.mercurymarine.com/portal/p ... ema=PORTAL
The site includes a Prop Calculator as well as contact info.
Here is the link to the Mercury Prop Calculator:
http://sites.mercurymarine.com/portal/p ... ema=PORTAL
Here is another generic Prop Calculator:
http://www.propline.com/Select-Boat-Pro ... s/main.htm
__________________