I think a day or two in Porto is a great idea, because it is a very beautiful (and now very trendy) city.
But as far as acclimating goes, the three most powerful antidotes to jet lag are aerobic exercise, fresh air, and sunshine/daylight. So whether you are walking or seeing the sights, I think the important thing is to get out and about during the day as much as you can. Pushing through the urge to take a long nap during the day works wonders.
After many years of taking a rest day before starting my camino, I got this advice from a forum member and tried walking on the day after arrival. Worked great for me. I try to get to the city where I’ll be starting from no later than early afternoon, spend the afternoon doing pre-camino stuff like getting a SIM card, getting some food for the pack, and climbing up to the castle if there is one. Early to bed and up for walking the next morning by 7.
I know that may be too hectic a schedule for some, but the important thing is to spend that arrival day out and about as much as possible. Since you’re arriving at 11 am, you will have most of the day to try it out and then see how you feel in the morning. But as I said, I would never turn down the opportunity to spend a day in Porto, it’s really a wonderful city.