Is a B+W +5 close up lens the same as a Nikon 5T
No, besides the fact that the 5T and the 6T are both dual-element diopters and the B=W 5+ is not, they also have a different magnification. The 5T is about +1.5 dioptre and the 6T is almost +3 diopter: when stacked it'll be approximately +4.5 dioptre which is almost the same as the B+W 5+ which is.... 5+ dioptre. to give you some idea: with the 6T (+3) on the 105 mm you can reach a maximum reproduction ratio of 1.5: 1, so my guess will be that you reach nearly 2:1 with B+W +5 diopter
I own both of them (5T and 6T) and they both work fine and i have no complains about the quality. With the 6T i experienced quit some difficulties with AF when used in cold, murky, low-light conditions, but fortunately Hugyfot recently developed a two piece manual focus gear for it, that exactly fits their standard flatport with a 25 mm extension ring (but the maximum fatness of the lenses for their ports is reached now...). I tested the prototype and i'm very positive about it, it's very convenient to help the lens a little bit to get near focus of your subject to prevent hunting of the lens.
cheers, Udo