There will be no effect, as you already gain 100% of the feudal tax (see SevenSidedDie's answer). The tax revenue depends on the size of the holding, but is otherwise fixed. The taxation law only determines how much of that cake goes into your pockets rather than those of your vassal lord.
You can, however, raise the city tax or church tax if you are looking for more income. The latter is especially useful, if you have free investiture, as you can make all your mates bishops, so they will pay tax to you rather than the pope.
It's also worth noting, that raising the tax will lower the vassals opinion of you. This might mean that they will grant you a smaller levy, only pay their tax in part (!), or be more likely to rebel. Some times that doesn't matter (say, if you're really popular and/or possess a huge, bloodthirsty army), but if you're new on the throne and you're barely holding your realm together, perhaps it's better to keep your vassals happy ... for now.
The last point is that of ineffective taxation, since you mention that you have personal control over your entire realm. It might seem better to keep all (or most) the holdings for yourself, but when your realm grows over your demesne limit, it will lead to ineffective taxation. Basically, every holding above your limit leads to a penalty. Often, the gains from those extra counties are greater than the total penalty, but if you get too greedy, you will greatly suffer. It's possible to get a 100% tax penalty on yourself if you go 100% above your demesne limit.
Wikia has a good explanation on how tax is calculated.