Hey Nick, it's a nice problem to have.
I went from an S to a 400.
I'm pretty sure I posted a comparison after a few weeks of ownership somewhere, but can't for the life of me find it.
1. The power and weight difference is noticeable.
2. The ride is not as compliant, and the S was definitely a better mile-muncher
3. The steering is much more direct, and the better for it when *cough* progressing. Less bump-steer.
4. Ingress/egress is MUCH better. I still can't risk a mini-skirt, though.
5. The interior felt more luxurious/ bespoke in the S, but the controls are much better in the 400 and there's no "airbag of doom" to worry about. I do feel that the 400 cabin is more spacious. Oh, and the HVAC is a marked improvement.
6. The Sparcos are not as comfy as the Recaros
7. I miss the change lights, but the small digital speedo at the top of the binnacle is a real boon.
8. You still can't wear polarised sunglasses and read the display.
9. It's cheaper to tax
10. Fuel economy is about the same
11. I definitely feel that the gear change is an improvement - purely subjective - just feels better.
12. Tyres last longer - although that is largely due to the switch to the Michelin tyre, which are excellent!
13. Noise. It's always in sport.
14. My S was nudging 70K miles. A newer car with the balance of warranty and not more than delivery mileage just took that niggling doubt, unfounded or not, away
15. I loved my S. A lot. I love my 400 more.
I get that you wouldn't want to follow the Herd, but that 410 of @bosshog is no ordinary yellow car, and it already has the Sparcos.
Edit: When I talk about Sparco's in @bosshog's 410, I mean that they're Sparco's as in a 400, not the cf seats.