There are almost no military records to be found about them, except the way they died
Kong Rong seemed to fear them though:
"Yuan Shao's lands are broad and his troops are strong. Tian Feng and Xu You are wise men to plan for him, Shen Pei and Pang Ji are loyal ministers acting in his affairs, Yan Liang and Wen Chou are brave generals in command of his troops. Here are serious problems.
Xun Yu counters Rong though:
"Yuan Shao has many soldiers, but his government is not well-ordered. Tian Feng is stubborn and insubordinate; Xu You is greedy and ill-disciplined. Shen Pei is self-opinionated and lacks original ideas; Pang Ji is too adventurous and independent. People like that find it hard to co-operate, and they will certainly disrupt his councils. Yan Liang and Wen Chou have the bravery of common fellows. One battle will be enough to deal with them.
In his reply Xun Yu talks down pretty much every capable man in Yuan Shao's army though (then again, Rong is positive about all of them). In any case, they were "known" among the enemy ranks.
During the battle of Guandu Yan Liang was sent to attack Dong commandery. At that moment Yuan Shao's advisor Ju Shou talked to his warlord about Yan Liang:
"Yan Liang is careless and impatient. He is brave, but he can-not manage alone.
That's 2 people who say he was "brave" (whatever that means) and one who says he was not. One of the people who does say he was brave, Ju Shou, probably knew him best though, so I guess we can assume he was "brave". On the other hand, he was talking to his warlord Yuan Shao though and maybe did not want to talk down the best generals he had (left) so he added the word "brave" (how valueable is being brave?) in his overall rather negative comment.
Anyway, it's very hard to say just how good Wen Chou and Yan Liang were. It's possible to check Xun Yu's opinion on Shen Pei, Tian Feng, Xu You and Pang Ji though and if you think he's wrong about them, he might've been wrong about Yan Liang and Wen Chou too and then the overall opinion becomes a lot more positive. Personally I think they were good generals and Yuan Shao placed a lot of trust in them by appointing them as frontier generals in such an important battle, but they may have been sub-officers in previous battles because they've never been mentioned before (yet did became known as "brave"). As was said above Qu Yi was probably better and Chunyu Qiong also might've been (he was a former Colonel of the Western Garden for the Han Dynasty, so he has been around for a while and had more experience as a high ranked officer commanding troops).