Some general 'boot thoughts':
Whilst many walker are still seen in heavy leather boots with thick wool socks, even in summer, over the last 10-20 years there has been a gradual move away from this thinking. It's part of a general move towards lighter kit in general, not just boots. However, keeping things light is especially beneficial with footwear as the pendulum motion when walking makes those grams weigh a lot more on your feet than when carried on your back. I still have a pair of Scarpa leather boots, and although they're pretty light for what they are, I still hardly ever wear them. An ideal boot or shoe for any person is one which is comfortable over long distances. Many things contribute to that, including; fit, cushioning, weight, breathability, warmth, support and waterproofness. It's often tricky to get all of those things into one boot/shoe. For example, a shoe will usually sacrifice warmth and support for breathability and lightness. Many walkers are even sacrificing waterproofing after watching fell runners skipping past them on the mountains, and there is an argument that extremely light and thin shoes are fine to walk in when it's wet because they're so breathable and they dry so quickly. If you're prone to turning an ankle, then you might not want to go down that route though.
On the South-west coast path you'll be basically going up and down, then up and down for over 600 miles. But the path is generally in good conditions and you're unlikely to encounter any serious deep mud. I'd want very light shoes for that type of walk. Goretex is helpful if waterproofing is important to have (beware cheap alternative membranes though, they often don't breathe well at all). Salomon make some decent light shoes, as do Scarpa with their Oxygen and Zodiac ranges. Meindl are good for those with wider feet, as are Merrell. But don't completely rule out some very lightweight trail-runners like the ones made by Innov8 - they're not waterproof but they'll dry really quick and you can always take a couple of pairs of spare socks to change into.
I remember sitting and chatting with a guy in Scotland last May. The weather was baking and his feet had gotten into a right state. I wasn't surprised since he was wearing heavy leather boots with a pair of thick wool walking socks and liners inside them. His feet had swollen because they were so hot and were rubbing inside his boots. I noticed he kept glancing down at my footwear - basically a rugged version of a pair of trainers and a pair of very thin merino socks.
Sorry for rambling...but that's what we do, isn't it?