Does anyone know how much this is taken into consideration for Tax LL.M. admissions?

In your situation I would think not much. You said the bad grades you got were in engineering... which has absolutely nothing to do with tax, and you've done very well in your tax courses. I would think the admissions committees would be smart enough to realize that doing poorly in engineering (which I think most of us tax people would) will have nothing to do with your potential to perform well in a tax LLM, and would give much greater weight to your tax grades. Now, if you had nearly failed in an accounting undergrad and had taken only bar-required courses in law school, then I'd say you'd have a problem!! But you made it to law school and did well there, took the right courses, and your undergraduate degree shouldn't hold you back. Not that I'm a committee member, but that makes sense.