Just wanted to share my experience with having the Spencer Seat mod done to my 919. I am in no way affiliated with the vendor - Just had an almost unbearable seat made new again.
Bike - 2004 Honda 919
Mods - Progel grips and Renthal High bars
Gripes - The seat was angled up too much. Causing me to slide forward under general riding unless I gripped the tank with my knees all the time. Was good for about 1/2 hr of highway riding until my rear needed a break.
I looked into modding the seat myself but based on foam costs and the time I'd have into it doing it 3-4 times before I got it right; so I decided against it. A local upholstery shop quoted me $200 to have it modified.
I settled on the long distance option for just the rider portion, sent an email with some pics of me and a description of my concerns. He said for the 919 that the long distance option is a must due to the thin foam in the 919 seat.
Shipping the seat - I bought a 12x12x40 box from a local moving/storage place for $5, packaged it up and cut the sides down to fit tightly against the box. Fedex was $15 each way. Took 3 days to get down there.
Turn around time - He emailed me on a Friday to let me know he had received it and shipped it the following Monday.
Impressions - Look - Completely stock appearance
Feel - feels alot like a memory foam type of material - comparing it to the passenger area you can see it takes more time for it to rebound after pushing into it, plus a larger amount of foam deflects when you push on it with your finger. This leads to much better support.
I've done (2) 1/2 hour rides with it so far. I don't move forward at all anymore and it felt great.
I'm very happy with the service, price and performance. I'll comment back after I do a few rides to lansing,GR or detroit with it.