Compared simple google search results with plugins off on both (since I can't get Flash player to work on Liquid ICS for the life of me) And yes there was significant differences in the smoothness of scrolling/zooming and panning.
Both phones are running stock frequences of 1024mhz and both were using the stock browser.
Like I said casual people won't notice these things, just like I expect a significant portion of the Android community won't notice the difference going from 30 fps to 60 fps in Jelly Bean with Project Butter, but people like me will notice and can appreciate it.
As for your argument that Sense will make the system lag vs AOSP, hardly. It takes up more memory, and chews through minimally more CPU cycles, but I tracked CPU usage using ICS's built in developer options in Settings, and I'm only seeing 7% CPU usage idling at my homescreen under AOSP with Nova with the Phase Beam live wallpaper.
Flip back to Sense 2.1 on my wife's TB running Sense 2.1, and guess what, 3% CPU usage average over the last 5 minutes using SetCPU logging. So your argument is invalid.
ICS will make a dramatic difference, no matter whether you're running Sense or AOSP, the ability to pick up on these improvements all comes down to how tech-casual you are. Me being a computer IT for the last 10 years and building computers for the last 15, also being heavily involved in game design and 3D architectural visualization, as well as software development, I have keen eyes for these sort of changes. And they're there, and totally worth it.
It seems you too notice some of these things, as you claim Sense 3.6 looks drastically different to Sense 2.1. I must remind you that people like us are a minority in the bigger picture of Android. The average HTC user is more like my wife. She uses her phone for calling, texting and listening to music etc. Simple things. If I showed her a Sense 2.1 ROM and then a Sense 3.6 ROM, she'd look at them both and say they look the same, or she doesn't care about the differences. THIS is what I mean by casuals not caring for the upgrades. It simply doesn't matter to them. And this is what HTC banks on, and is correct in assuming, as for their reasons to delay upgrades and hold devices back. Because simply too many people do not care for the differences.
My dad's Droid Razzr Max was upgraded to ICS. I asked him "did you get the update on your phone? It makes it a lot better."
He replied, "what update? it's the same thing I didn't see any updates."
Checked and sure enough, he was on ICS. These are the common people who use these phones. They simply don't know or care to look for these differences. Unless OP is into ROMing and themeing his phone, I highly doubt he's going to notice or care for the difference when his TB gets ICS.