I have an LTR Retirement. As I understand it the retirement version puts you in a tax exempt catagory while the other LTR Visas allow you to pay the reduced rate on Money brought into Thailand. Obviously money earned in Thailand would still be classed as taxable in all versions although I think this is at the lower rate you mentioned (I could be wrong on this).
When I was applying for my LTR my agent showed me the Listing in the Royal Gazet which approved the tax exemption for this class of visa and I have seen several confirmations of this by reputable accountants since. I checked with my agent in late December and she assured me I don't need a TIN and am tax exemped.
I am surprise that more people don't know about this although you obviously have to have a pretty good income from investments to qualify.
With regard to Thailand entry I can relate, as the immigration officer told me on my last entry. that it the first time he has ever seen this Visa and he also had to go and look it up!