I bought full bank space but if i become a life subscription member as its part of the membership would i get a refund?

No. I had purchased bank and inventory slots before I went LTS, and no, there was no refund.

C-Store Bank/Inventory slots are not the same as account type slots; You will retain what you purchased in addition to the slots gained from account type;

For instance, I have somewhere around 108 inventory slots on my main.

I confirm the above answer as well. Up till the time you become a LTS any money spent before on bank/inventory is not refunded to you...sorry

Shame they should see you want to be a lifer and any money you spent on bank should be converted to lockbox keys

It's as atatassault says. If you buy bank/inventory space before you upgrade to Gold/LTS, you will get total of everything. Your Gold slots PLUS the ones you bought. I can confirm this as well. You can continue to buy them as a gold member to add even more.

I bought the max number of bank slots i didnt realise there was more

There are, just not available to Silver players.

Same goes with Bank and Account Bank storage. Also, note that you keep this Gold-only storage on your pre-existing characters even if you go back to Silver.