ADSL2+ upload speeds

[#1]

Following the recent upgrade of my local telephone exchange, I'm going to be upgrading to ADSL2+.  I've spoken to PN support today, who advised that I should see an increase in upload speed, up to double what I currently get (448 Kbps).  This surprised me, since I thought ADSL2+ was 1.2Mbps upload speed.  The agent did say, however, that another option was that I could request for the upload speed to be uncapped.  Does anyone know what this means? Would uncapping it give me the full 1.2Mbps?  If so, why is it capped in the first place?
Thanks,
Gary

[#2]

The agent is getting confused....
Moving to 21CN/ADSL2+ does not automatically increase your upload speed. While your download speed will be whatever your line can manage, the upload remains fixed at 448Kbs. To get it faster you need to request your upload speed to be uncapped. Then, like download speeds, you will get what your line can support which can be anything higher than your current speed but unlikely to be near 1.2Mbs (which is the theoretical maximum on Annex A I believe, Annex M can go up to 2.5Mbs but this is not available from Plusnet).
The reason it is capped is to improve the reliability of the connection. Uncapping means higher frequencies are used which are more susceptible to noise and interference, which can sometimes affect download speeds as well. See here for more info.

[#3]

Thanks, Peter.  I have a good service on ADSL at the moment (8128 upstream).  Do many people experience issues with stability if they uncap the upstream speed?  I could really do with a faster outgoing service and it's tempting to order the upgrade...
Gary

[#4]

Go for it - before I joined PlusNet last year my upstream speed was uncapped, getting typically around 960kb/sec up. When I signed up with PlusNet, my upstream was capped, and I didn't realise this was why my connection was performing so badly - got it uncapped and have had no problems since.
In any case, if you do find it makes your connection worse, you can just ask for it to be capped again.