In a DB scheme your max lump sum is 150% of Salary subject to having a minimum of 20 years service. However Salary is your gross taxable including bonuses etc. however your pension scheme is probably based on basic salary. So using AVCs to increase your retirement lump sum is very tax effective.
If you have less than 20 years service then your max lump sum is reduced. But even still there may be a gap between what your scheme pays and what are Revenue limits. Again using AVCs to fund the gap is sensible. And if that means all the AVC fund is so used, that's ok.