I regularly tow a TT that weighs 10k. Personally, the downshift works perfectly. I generally tap the brake when I know I am going to slow down and she immediately downshifts. It's like we're dancing. Haha
Also, I may or may not have used it when putting around town and that downshift gets my magnaflow to sputter a teeny bit when you coast down to 20mph. Music to my ears.