My boat (21' Edge tunnel) with a 454 goes from 74 mph @ 5600 rpm to 89-90 mph @ 6800 rpm on nitrous. Just make sure your motor is built to take the strain and that you have adequate fuel pressure, you may also need to back of the timing. Mine is set so that a separate electric fuel pump kicks in when the nitrous is activated. You can go with a smaller system and alleviate a lot of the problems if you only need to gain a few mph.
BTW- It is instanteous power when you kick it in. The downside is that nitrous where I live is $3.50/lb and a 10# tank is probably only good for about a minute of use, but I have a big system.
JB