6" offset box for flex liner

Does anyone use these:
http://www.chimneylinerdepot.com/store/61/product/6-inch-Adjustable-Offset-Insert-Adaptor.html

A fellow here had a problem and found these and had some questions about them. My question is do they leak? This is the item I am looking for to give me " more stove". They just look like it would leak like a sieve! Any information would be wonderful. Oh and I am not worried about draft as mine is quite strong....

The elbow will be a problem because my liner comes straight down and this thing would.........unless I use 2 elbows?

I just looked and the elbow(s) are not going to work. However cleaning can be a slight pain but I can get my shop vac hose, without the baffles in, up in the area no problem.

See the 6" will give me the ability to insulate behind and on the sides better while lining the fireplace firebox with sheet metal. I believe this will give a huge heat gain!?!?