I am very pragmatic about things like this and this is a mod worth doing
, but don't expect to feel a difference before and after
Lightening anything on the motor does not release HP, it only allows quicker acceleration due to less weight, and 800g is not much - lightened flywheels are in the order of 5kg lighter
Making the pulley smaller will drive the components slower yes, but things like the alternator draws as much HP power as it is required to produce, very little to do with the absolute speed it is spinning at. E.g. a fully charged battery in without any lights, radio, fans, etc on with a fast spinning alternator will draw less power than a battery being charged, lights, radio and fan on with the alternator spinning slower.
Let us assume a linear relation, and following performance - How much power (HP) do the components on the engine belt generally use? - Motor Vehicle Maintenance & Repair Stack Exchange
AC: 4kW (5hp)
Alternator: 5-6kW (7-8hp)
Water Pump: 10kW (13hp) - cambelt driven, thus irrelevant here.
Power Steering: 2-4kW (3-5hp)
Total = 18hp Assuming linearity then 30% = 5hp
EDIT: to come back to the lightening rotational mass question - it will only help you in-gear acceleration, not your take-off acceleration. Your take-off acceleration has more to do about the amount of energy stored in the rotational inertia - you are taking some away, thus coming off the clutch, the engine will slow down quicker than before and in essence your clutch will wear less, the once the engine is fully engaged with the wheels, needing less energy to go into rotational inertia - more energy is available to go into forward moving energy (but we are talking margins here with 800g at stake)