The size comparison picture at the end of your review appears to be off. Sigma is measuring ø 86.4 × 182.3 mm, Tamron ø 86.2 × 199 mm. The Tamron in real has the same ø and is longer. I know there's a hood on the Sigma but in reality both lenses feel very much the same. Even the weight difference is only 25 grams.
Details are different, though: While the Sigma can stop down to f/22, the Tamron maxes out at f32 (at 100 mm) and f/45 (at 400 mm). No idea when such an aperture might be helpful except the usual waterfalls? The Tamron does feature a lens collar, which is then mounted on the metal part of it's barrel (thanks Tamron, for doing this right) and has, no, is an Arca type plate.
And the focus limiter is one of the coolest I know. It works like all the others but can also be set (via Tap-In console) to a mode where the range of distance depends on the chosen limit AND on the position of the distance ring while activating it. Say, limit is 3 m and focus ring shows 2 m when switching to focus limit = range is 1.5 - 3 m. Same limit, but focus ring shows 5 m when switching "Limit" on : range will be 3 m - ∞.
Fuji engineers, are you listening? With your expensive, non-linear focus by wire, small tripod plate, zoom-creeping 100-400? Go and send a trainee over to Tamron, they know how to do things!
I look forward to read the Tamron review but I keep thinking in the (affordable) tele-zoom range no other manufacturer comes close to Tamron. Not only the performance (aside from slow lens) but the details really make me happy.