I also purchased a Britomart which had a tight spot that did not improve after 2 hours of running in. I exchanged it for another one (I can't recall ever having to return a model before) and it was better, but still seems to occasionally have the same issue (at the same point of rotation as well). I do hope a decoder will improve running, but generally a loco should not have to rely on one for acceptable running. I can't find the Zimo decoder in stock anywhere (I gather the Bachmann one is the same, but at a higher price) so the jury is still out on this.
I appreciate these are small locomotives, but my expectations are high after seeing how my Bachmann N gauge Percy performs which is a similar size. I am now hesitant about the new Penrhyn Hunslets, as videos online also show mixed running qualities, which is a shame as I'd really like the tender versions when they inevitably happen.