Qumran and the growth of the Psalter
According to Peter Flint we can discern three literary editions of the Psalter: Edition I (Pss. 1/2 – 89), Edition IIa (Pss. 1/2 – 89 and 11QPs^a), and Edition IIb (Pss. 1/2 – 89 and Pss. 90-150). After looking at the Qumran MSS Dwight D. Swanson states that there is manuscript evidence for at least three Psalters existing simultaneously in late Second Temple Judaism: an MT-type, a Cave 11-type, and an LXX-type and suggests that it is not unreasonable to conclude that there could have been more. The MT Psalter then appears to be the latest edition of the Psalter and, he argues, that the MT-Psalter reached its final form in the late first century C.E.*
* SWANSON, D. D. (2005) “Qumran and the Psalms”, in David Firth and Philip S. Johnston (eds.) Interpreting the Psalms: Issues and Approaches. IVP. pp. 259-260
תהלים 
It seems to me inappropriate to argue from Qumran to broader issues of canon for two reasons. First, it is clear that Qumran does not represent normative Judaism. It is rather a splinter group whose commitments and practices were not likely widespread throughout the JEwish world. Second, it is not clear what the function of any one scroll, even a biblical scroll, may have been. 11QPsa may or may not have been a “canonical” scroll or even have originated at Qumran. If it originated at Qumran, it may have functioned like Hodayoth. Questions of canon may be involved in these issues, but they may not. Isn’t it a bit early to be drawing these conclusions?