I don't think it's a mistake at all. When I shot a Tavor 7 in .308, it was a pleasant enough experience. Biggest complaint I've seen is the accuracy has dissappointed people.
I do have to ask - have you considered the Desert Tech MDRx? From what I can tell, they've worked the bugs out in the "x" version. Price point is comparable. I like the MDRx because it has a few better more modern features, and the ability to switch it from .308 to 6.5 CM to 5.56 to .300 BO is kind of neat. And the ability to switch shoulders and shoot either side of one's face is a pretty nice feature.
If it were me, I'd get the MDRx over the Tavor; but I the Tevor-7 is a good gun too. My own logic and criteria though, is the point of the heavier round is to increase effective range. There are few settings where at 50 yards .308 would have been that much more effective than 5.56, in my opinion. But at 200 yards+, that difference will really start to show up. Which is where accuracy starts to matter. And on that note, if my goal is to hit stuff hard at distance, then 6.5 CM start looking the more interesting. If my goal is effective bullpup for sub 200 yard work, I view 5.56 as just fine (obviously not everyone will agree). That's why I never really looked that hard at Tavor-7, it wasn't available in creedmoor, and it wasn't sufficiently accurate to justify the $2k pricepoint for hitting stuff at distance. Obviously, others have other criteria, and it is a pretty bad-ass gun, so definatelly not a mistake to get, if you want a harder hitting 100 yard .308 bullpup. Though... I think MDRx does all that, and more.