I swapped from carbs to Merc EFI on a set of415hp 502s a few years back. Yes, it starts much better. The throttle response is better, and it burns a little less fuel, and idles solid as a rock.
But......It was a lot of work and expensive. I struggled with lean running, vapor lock, idle issues, and a failed injector and misc sensors. I finally have it running perfect now after adding an o2 sensor and buying the MEFI Burn to custom tune the fuel and timing curves to match the way I use the boat. For me, it was worth it because I like to tinker with this type of stuff, so if you are looking for a challenge and enjoy tinkering like I do, then have at it. You will have a blast. But if you think that you can just spend a weekend for the install and you are trying to save money through fuel savings, I would strongly suggest that you pass on the project.

Steve

Nope. The 502 mag and 454 mag were 115LSA.

They(454 and 502 Mag) are actually 115.5*. The 350 mag is on a 109*