As long as you're within the Annual Allowance (AA), you'll get full tax relief on any pension contributions you make. This means that the cost of contributing £1 of pay is actually only 80p if you're a basic rate tax payer (or 60p if you’re a higher rate tax payer).*
If you leave the Scheme any AVCs you’ve made - including any through Pension Extra - will remain invested for you by the Trustee. The additional benefits you’ll get when you retire will depend on the value of your AVC fund and the cost of buying those additional benefits.