A larger servo won't give you better brakes it will just reduce the amount of pressure you have to apply to the pedal to acheive a certai amount of pressure at the slave cylinders. Its not that many years since we were all driving cars without a servo at all.
How do I put this - we have a fair number of spirited drivers on here inclding those that have fited larger wheel/tyre combinations and no one has really complained aboutthe brakes to my knowledge - sure I've managed to get mine to start fading when pushing on in the dales but only after sustained use, and that was with whatever ceapo pads were in when I bought it.
If you really feel you need better brakes I'd first suggest;
1) making sure the entire std sustem is working at its best, disc condition,sliders free, drums adjusted and clean, proportioning valve free to move etc
2) then fitting uprated brake pads EBC 6000 series (green stuff equivalent for vans and SUVs) part code DP6947 - about £50 inc P&P
3) Personally I'm not a big fan of grooved and drilled discs etc but some people rave about them but good quality discs are certainly worth the money be best to fit them at the same time as fitting new pads of course.
It may be possible to fit larger discs and better (4 pot maybe) calipers to the front and discs to the rear but I've not heard of any easy conversions, it may be possibe to use the rear disc brakes off a Patrol as they share te same base axle on some models but tbh I really don't feel the need and I have as much (probably more) extra inertia from my tyres to slow down, 32/9.50/15 now and had 265/75/16 on before.
