It's up to you whether you want try port forwarding or not...it's not anything that's going to stuff up your computer (the static IP address isn't in any way linked to your normal IP address...it's just set up so that your router can forward the port & recognise your computer). I couldn't find any info about whether your router supported UPnP, so I didn't suggest enabling that

Your random download speeds might just be down to TalkTalk...they have a FUP which will throttle p2p download speeds.
Find answers to your questions about TalkTalk in Help & Support

And this suggests that they're traffic shaping 24 hours a day now, rather than only during peak times.
TalkTalk Members - Traffic Shaping and Bandwidth Capping

Did you try changing ports?

I clicked on Tools -Options - Download and changed my speed from unlimited to about 10 KB, clicked on Apply and saw my download speed go to 10. Then I increased it to Unlimited (this is using the slide bar) and Apply. The speed returned to its normal of about 170kb.