To be fair Virgin have been great for me and I have been with them for will over 12 years.
Just these annoyances I want to try and solve, perhaps some settings in the modem/router settings or something minor that can be done just to improve it!
I've been with them through the NTL/Telewest/Blueyonder eras and their broadband has always been rock solid. In fact it's probably the one thing that has kept me from leaving during earlier eras, especially when Virgin first took over when the company was infuriating in many other respects. It did seem to be the case that some years ago Virgin Media upped their game in various areas compared with their former failings - and noticeably so by comparison with the infuriating Virgin Mobile - but in recent months the broadband seems to have gone over a cliff for some reason.
T3'a are okay if they rise slowly. If the numbers are in the hundreds over a few days, then that is a problem. So, just keep an eye on those.
The power levels are close to the bottom of acceptable range, they should be between -6 and +10, so you are borderline. If there are any attenuators on the line, you could try removing them and see if the levels rise. If not, just put them back.
Agree about the WiFi, it's not the best for speed testing as it's reliant on the environment at any given time. If you have an Android phone, get a WiFi analyser app and walk about the house and check for best WiFi spots.
Have you updated your Linksys firmware to the latest version, as this can improve security and performance and I would recommend you do this.