As far as velocity goes,
I don't believe you will gain any velocity because you are increasing your mass, but still only applying the same amount of energy to it to get it moving. You may gain penetration because once you get your arrow up to speed, it will retain more energy because of it's increased mass. Think of it this way, If you try and throw a cotton ball, You can get it going real fast but it doesn't possess the energy to keep it going. Likewise, if you throw baseball it will travel farther because it has enough mass to carry it through the air.
As far as weight tubes go, I really don't know anyone that makes them for carbons and I think the more important issue is going to be shot placement. You can bring down the largest game with a correctly placed shot and that should be the ultimate goal. If you aren't sure if your shot will hit where you want it to, you may be either rushing the shot or not have a clear enough shot.
The other thing about weighting your arrows is that you may also be changing the arrows ability to flex (spine)which may make it shoot differently.
Not saying it's not worth investigating, but there's a lot more to some things to consider than what we first think of.