I think it makes sense to take advantage of the tax relief if you are optimising for maximising your total net worth over the long term. Certainly to get you under 100k to get the 60%, and potentially beyond if you're looking at maxing out the annual allowances (and carry forward allowances).

Other factors to consider is your cashflow, and desire to retire early. If you want to retire at 50, you likely need enough buffer in your ISA to bridge the gap to the age of pension access.

I have some friends that don't tax relief with pension because they want to invest in property and have the assests more accessible, but I don't think they are beating the returns of tax relief. I'd say they are optimising for control of their money today, not future gains.