music is dynamic so that load will never be consistantly maxed out.
A bigger alt is overkill for this system imho. I've seen plenty of installs with much more power on stock alts without bad dimming.
1. do the 'big 3'.
2. run a seperate ground cable to your amp (alot of dimming probs come from bad grounds)
3. get a good batt to replace your stock.
If you still get bad dimming, then
4. get a second batt for your trunk.
Note: You can upgrade your batt without upgrading your alt. But you would be foolish to upgrade your alt without also upgrading your batt. Keep that in mind.