I Honestly think 3 x 12 hour days would be better than 5 * 7.5.
I am a fan of 5 x 6 hours and 4 x 8 hours.
Documentation, meetings, replying to emails answering someones question.
When get to do some coding its not usually especially complex. Occasionally it is, but I imagine having longer days would be easier to get 4 hours "in the zone", something which happens rarely at present.