I have 3 15 gallon Blichmanns. You will not be able to do 10 gallons of barley wine in the mash tun. You could do an 8 gallon recipe, I think the OG was around 1.120. Everything else will be fine for 10 gallon batches. I just brewed a 1.093 with no problem for a 10 gallon batch. For boil volume, depends how much pre boil you need. I fill my BK up to 12.5-13 gallons, as long as I watch it, I don't worry about a boil over.
As far as the thermometer placement for 5 gallon batches, it is a little bit of an annoyance. The HLT is too low for 5 gallon batches, the MLT is fine once the grain is in it. The BK, not a huge deal since you are just boiling. Just have an extra thermometer for 5 gallons.
The sight glass is the best feature, I think. The thermometers and ball valves are very nice as well. After purchasing the blichmann kettles, I decided that i am not a fan of weldless fittings. I do not have any problems with leaking or anything, I just would prefer welded fittings.
the pots are pretty thin, but I doubt there will be any problems with the pots breaking. The handles on the lid and the pots seem to be cheaply attached. I often carry the pots around with water and grain. There is warning that states not to do that, but so far I have not had any problems, but I worry every time. I did have a lid resting on the handle of the pot once, and I bumped the lid off. I picked the lid up by the handle and it came off. I contacted Blichmann and they sent me a new lid right away.
Customer service for Blichmann seems to be top notch. Every time I have had a question to e-mail, I get a response shortly after.
You could probably drill your own hole for the new placement of the thermometer yourself, for 5 gallon batches, and have a weldless plug for the hole you are not using. Having an external thermometer would be easier, but I understand why you would want the option of moving the thermometer for 5 gallon batches.