I've made the transition you're talking about, currently preferring a tent (Big Agnes Seedhouse SL1) to a tarp and bivy (my choice was the ID Salathe and a 5x8 Siltarp.) I tend to gravitate between the two, depending on time of year, expected weather, and length of trip; however, the tent is probably my choice at least 75% of the time. As far as pack weight, the SL1 weighs within a couple of ounces (I'm not even sure which way) of the Salathe/Siltarp combo, so it's a push for me. I've tried lighter bivy sacks, but never found one I liked better than the Salathe: the open-to-the-waist weatherproof panel is fully backed by a similar bug netting panel - a real necessity to using a bivy in the bug-infested humidity of Ohio, Kentucky, and Indiana in summer.
I noticed you were wondering if the money you were spending on a tarp and bivy would be better spent on other gear. MLD makes excellent products, well worth the money, but I see your point. You also indicate that you'll save over 4 pounds by making this switch, which makes me wonder what version of the Seedhouse tent you're using. Assuming that the tarp and bivy will weigh at least a pound, the only Seedhouse tent I could find was the Seedhouse 3, right 5 and a half pounds, packed.
So, here's a middle-ground suggestion. Since you're toying with the idea of a bivy sack, you apparently don't need a 3-person tent. A Seedhouse SL1, at just over two and a half pounds, will save you about 3 pounds, and leave you $200 to put toward the other gear you mention. Since those items should also save you a couple of pounds, that might be a "most bang for your buck" approach. Also, since you already know you're comfortable in a tent, replacing your current tent with a lighter one means you don't have to try to make the adjustment to being comfortable in a bivy. (Am I correctly inferring from your post that you have at least a little apprehension about switching from a tent to a bivy?)
A bivy does take some getting used to. It's going to be a lot more confining, and if you're using a quilt, you may find it a little awkward to use it inside a bivy. (I frequently use my WM Mitylite bag in quilt mode, and found that it was really hard to adjust it as I rolled or moved around in a bivy; it was particularly hard to let a leg hang out from under the bag.) There's also not much space for other gear inside a bivy (not a huge problem if, like me, you don't carry very much miscellaneous gear and you don't keep your food inside your tent with you.) A bivy really can't compete with a tent when it comes to general livability.
Don't get me wrong, bivies are very nice. They let you camp in some places (like rock ledges) that a tent won't fit. But the current generation of tents, like the SL1, have pretty much eliminated the main reason I switched to a bivy: sleeping under the stars. With a bivy, on a clear night, you don't need a roof; the open feeling of sleeping under the stars is one of backpacking's greatest pleasures. Until the last few years, most tents had solid fabric walls, so you were visually closed in even when you didn't have the fly attached. However, tents like the MSR Hubba and the Seedhouse SL1 have all-mesh walls and ceilings. Lying in one of them, with the fly off, gives me the exact same sense of sleeping under the stars as my bivy ever did, with a lot more spaciousness.
A bivy/tarp combo is certainly a workable setup, but you may be able to accomplish more for the same cost with a replacement tent and other gear. Good luck.