Am now polishing things up to finally sit Grade 8 piano in the Fall. Am actually spending much more time on other repertoire while getting the old scales in shape and getting a bit of coaching for the aural. Of course, can't help but want to start planning out already how to work towards DipABRSM.

It will have taken me almost five years of fairly intense study (1-3 hours/day, 6 days/wk) to go from Grade 0 to Grade 8. As someone in my late 40's I am very pleased that a long-held dream is coming true and that I can now play a bit of piano -- but I want to keep moving upwards and onwards.

I have tried to glean as much information as possible from this forum and other sources to come up with my roadmap towards sitting for the DipABRSM in piano performance. This is what I have come up with:

- take at least two to three years and don't rush it; you & teacher will know when you are getting ready;
- spend much more time on longer works (complete Sonatas, etc.)
- spend remedial time on weak areas (in my case: Baroque/polyphony, memorization, not rushing in performance, accuracy on long, fast runs, relieving tension, avoiding keybedding, etc.)
- get technical apparatus upgraded to the next release through studies and harder repertoire (Brahms 51, Carmer, Czerny, Chopin, etc.)
- sit grade 6 theory to refresh theory knowledge
- explore exam repertoire and come up with 2-3 alternative programs to find pieces/program that best suit;
- analyze all pieces and be able to talk about analysis
- research periods/composers/pieces etc. of all exam rep being worked on; be able to discuss extemporaneously;
- sight read grade 5-6 pieces often and work a few up to performance standard asap;
- take chances to perform in public whenever they come along;
- work a minimum of two hours/day

Does this list make sense?

That certainly makes complete sense. Its good to see someone who has thought about the direction they are going, and what needs to be done.
I dont think that you necessarily need 2-3 years from Grade 8 before tackling Dip ABRSM. With the commitment you are demonstrating I'd have thought 12-18 months should be possible.
But yes its a good idea to get your theory up to date, and also think about playing whole sonatas and longer works. It is also a good idea to expand your knowledge of all periods of repertoire, by listening both CD's/radio, and attending concerts when you can.
I wish you good luck in your endeavours, and please keep us posted how you are doing. There are plenty of people on here willing to offer advice.

Your plan sounds great and so well thought out. I'd like to add one item that I found extremely helpful. If your sight-reading is not all you'd like it to be, find a piano duet/duo partner and meet regularly for sight-reading sessions. (Maybe your teacher has a pile of duet music that you can work through.) It also just happens to be lots of fun!!