The new seat is designed for their rec boats and daytourers which have wider, deeper seatpans than the performance models (like the Solstice series).
If the seat is already tight on your thighs then it's only gonna be worse when you wear winter layers and drysuit.
If you really like the boat IMO the best options are
a) lose weight/work out so that your hips are slimmer. You wouldn't be the first person to use a kayak as motivation!
b) remove the whole seat pan and either make or buy a seat carved of foam. Install it so that is is sitting ~3/4 lower than the extg seat and that'll give you more clearance.
Check out the Hot Seat kayak pad (self adhesing) ~$25 -$29. Comes in a Grande Size for big butts. (no offense) Made by Surf to Summit. Adhears right to extg. seat or to the floor of the boat which is what you could try. Use it and keep the extg backband, at least for now.
Used their Blue (Mild) Touring one for a couple of years on my first kayak and really liked it. If I wanted to augment the seat on my new Pyranha Fusion S (don't, so far) I'd use the Black (Chili) pad.
either option is a lot cheaper than another boat!