According to the online tyre calculators the standard size is 27 inches diameter overall & the size you propose is 28.91 inches diameter overall. That's around 7% incrase in diameter which will definitely affect your speedo. As for clearance, now you have the figure you can roughly measure up to see if there are any areas where it might catch.
205/80/16 Insa Turbo Rangers. They run VERY close to the spring lower caps but they do have 12mm of tread depth! I've had no issues with them at standard ride height and they give an extra inch of clearance, although they very slightly rubbed a few times on the plastic arch liners when fully compressed and the car was very heavily laden. They effectively make the gearing slightly higher although this actually makes the speedo correct (according to my sat nav), and as long as the tread isn't too off road biased could even increase your MPG! (lower revs at any given cruising speed...)
I've now raised the car 2" on lift plates to aid my off road driving, and I'm loving it! Loads of clearance and no axles or diffs to thump into rocks and stumps like the trattors!