Mate, he's pretty good. He doesn't have the punch that some more compact (i.e. shorter) props do, but the height (coupled with his strength) gives him greater leverage with offloads and the defensive wrestle.
Plus he does make a lot of metres and he's athletic so he can play 60 minutes if needed.
He makes a lot of overall metres per game, but his metres per hit up is rather mediocre, as I found in my research. Scott, Mannnah, Gallen and Fifita (if memory serves) all made more metres per run than Tamou.
Now, there are two predominant schools of thought in regards to making metres in rugby leagues, as my coach once said (and he modelled it on the NFL and their running backs):
(1) total metres per game; and
(2) average metres per carry.
In the NFL, they use an important metric called "yards per carry" (often abbreviated to "ypc"). Most NFL coaches would rather see their running back's ypc high, regardless of their total yardage for the match. The reason is simple:
Let us assume a running back makes 20 runs in a match, and makes 150 total yards. He's made an average of 7.5 ypc. Considering a down is 10 yards (allowing for no penalties or sacks), making 75% of the yardage required for a first down on average every time he runs the ball is solid work.
Other coaches prefer running backs who shine in the first category; they'd rather see a running back run for 300 yards off 30 carries. But these coaches think that, firstly, repeated runs will sap the defence, and secondly, quantity is better than quality.
Appropriating that into rugby league, you can have forwards who make good overall metres from lots of runs (like Tamou), and forwards who make less metres on average, but they make more metres per carry.
James Tamou is very good at making 15 runs a game or so, and making 115 metres a match or so. But then you might have a prop who makes 10 hit ups and makes 100 metres - at first glance, his numbers look poor. But he's making a solid run every time he touches it (on average).
In conclusion, I think that Tamou, given his sheer size, is overrated. He should be making more metres per carry than Mannah.