I have the same mash tun. It's good for most 5 gallon batches and can handle 10 gallon batches up to 4-5% (at which point it is full to the top). I would not use a cooler as an HLT because if the water cools too much you can't apply heat. I use a burner and pot for HLT.
Another problem is that there is heat loss when you transfer to the cooler. With a cooler HLT, you must transfer twice. I just think it would be harder to predict the heat loss.
So I like the mash tun but there are times when I wish I bought a 15 gallon mash tun for big 10 gallon batches. You can buy a propane burner and pot with valve cheaper than the HLT portion.
Also, this is set up for fly sparging and I love batch sparging.