Not sure I understand what a 'splitter' is. The I Plate is an additional piece of kit which I think you have to buy from BT. I have a NES 5 faceplate but no 'I Plate' as I simply didn't connect the orange wire : achieves the same thing. The better result from the test socket points to room for improvement in your internal arrangements (telephone circuitry, not the wife). Not sure how you know what level of errors is ok. I don't here, and one set of 'errors' (FES or some such thing) is purely because the line is interleaved and not 'fast'.
Incidentally, if you are on windows vista or later, all the web gossip seems to indicate that windows now does a fine job optimising settings (so no more Drtcp and similar tweaking tools). Unless your ISP says otherwise, I would set the router to mtu=1500 and leave windows alone.