retro fit servo?

Hi all,
I have just changed all brakes and cylinders on my non servo S III.
All has been bled from rear lhs, rhs, front lhs, rhs.
It still needs an elephant style amount of foot pressure to stop it! with the pedal feeling spongey and not bringing it to a stop very quick.
Does this sound like a master cyl?

This was meant to be a rolling restoration but i do need to stop the thing occasionally!

Would retro fitting with a servo be abig job? or worth doing?

Thanks for helping
James