Well admittedly I feel as though my knowledge of the details of memory management could be perhaps be improved to some degree, but I do feel that I have a decent grasp of the fundamentals, and I do of course absolutely agree that sluggishness relating to memory management would be a result of thrashing not simply utilisation. Now perhaps with your particular example system with 77% RAM utilisation you're experiencing very little trashing, but that doesn't mean the OP isn't; I do not believe that high frequency thrashing only occurs near 100% RAM utilisation, I believe (and perhaps I need to do more research into this) that Windows pre-emptively moves pages into the page file, keeping some RAM free allowing for quicker response to spikes in demand, and that the higher the utilisation, the more likely and frequently that thrashing could be occurring. Thus I feel it is entirely feasible that you could see a system with only 70% utilisation (which I consider fairly high) and yet with a large (relatively) amount of thrashing. For example, a user may have a browser with a large number of tabs open, and only 70% RAM utilisation, as here, and see thrashing because firstly, Windows is keeping the remaining 30% open in order to allow other applications to be opened quicker (as a measure to try and provide a better user experience), and secondly, the browser may be causing pages to be moved back and forth between RAM and the page files as tabs do stuff such as load new content via AJAX or continue buffering video, or simply as the user switches between tabs in the interface. That's just my speculation though. If I'm right, adding more RAM could result in Windows allowing a larger proportion of the browser's pages to remain in RAM, thus reducing thrashing and thus reducing lag. I do have a PC here used by other members of my family, which previously had only 2GB of RAM; this was never maxed out but utilisation was quite high. Increasing this to 4GB resulted in a definite improvement in performance, which I feel supports my hypothesis.
Of course I'm not stating out right that thrashing is absolutely the issue here, I may be wrong; we know nothing about page file utilisation and fault frequency on the OP's system itself to say such a thing and there could be other causes. The OP states that lag is noticeable at 70% RAM utilisation and the implication is that this is a result of an increase in the number of tabs, not other applications that are then consuming significant resources. Perhaps the additional tabs result in an increase in CPU cycle consumption by the browser; perhaps as you suggested there's an increase in HDD activity related to caching; both sound reasonable to me. Perhaps it's a combination. However my gut feeling, based on the fact that the OP has a recent Haswell based MB, and likely a decently fast enough CPU to go with it, and that the lag occurs at high memory utilisation, is that increasing the amount of RAM is going to result in an improvement. Regardless, I feel that the OP could benefit from the extra headroom.
With that said though, perhaps checking disk activity might be a good idea to be certain. I still feel that more RAM to give more headroom would be good, but of course if disk activity is high, whether from frequent page faults or caching, an SSD is going to be the best solution if the OP doesn't have one.
Edit: backing up I've noticed that the OP has actually stated how much RAM they have (presuming the link is the actual item, not just the same model with a different capacity), I was assuming the OP had something like 4GB. So you have 8GB. That's actually a pretty decent amount. What browser are you using. I ask this because on my previous system (which I replaced a few months back) I had 6GB of RAM and shortly prior to replacing it I was doing some research with a ton of tabs open in both Firefox and Chrome, and a few times maxed out both my RAM and page file resulting in Windows issuing memory errors. Chrome was the culprit, consuming a huge amount of RAM across it's large number of individual processes. If you're using Chrome, you may find that using Firefox instead will reduce your RAM consumption enough to negate any need for purchasing hardware. (Disclaimer, I'm a fan of both browsers, I'm not acting as a fanboy here).