IMHO, the Sportster seat mod is overrated. The standard S40 seat is just as comfortable (to me anyway), looks just as good, and takes a whole lot less modding to make it work.

That being said, the Corbin seat is like a whole different realm of existence. I sat on a Boulevard with the Corbin seat and it felt like the seat was born to be there. Perfect fit, look and comfort.
Of course, you DO pay for it.....but if I had the cash it would be a no brainer.

"I look at the pics of the Corbin for S40 and it looks like the driver is pushed forward more so than with the Suzuki factory seat."

That was my experience. I'm 6ft. The Corbin seat put me noticeably forward to the point that I was unconfortable. This was with the standard Savage buckhorn handlebars. I liked those bars, but with the Corbin, they cramped my arms too much.

Kenny, I'm 5' 11" and a smidge. The buckhorn bars are just flat out awful for a guy with 25 inch sleeves. I personally think they suck when it comes to low speed handling and agility no matter how long or short your arms are, but that is just my personal opinion.

At 5' 11" I dont feel cramped on the Savage now that I have the Bonneville handlebars on it. That being said, I'm not a "cruiser" guy. I come from the "standard" bike world. I'm very uncomfortable with my feet way out in front of me.

The Corbin on the S40 with stock bars didnt feel cramped to me, and I liked the small amount of lower back support. Again, your mileage may vary.

As for modifying the stock seat to be more like a standard seat, I'm not sure how you could do that. I'd probably start from scratch. I'd use 1/4" ABS for the pan because it is easily heat moldable. Cut out the general desired shape, buy a Honda standard seat "replacement cover" since I dont think the S40 vinyl is going to conform to the shape you want.
Heat the ABS with a heat gun or torch and start to mold it to the shape you want on the bike. Then add foam/gel/whatever filler and cover it. Using a Honda-stye cover like this would probably be easier than using the original vinyl.
http://www.ebay.com/itm/HONDA-CL200-1974-Custom-Hand-Made-Motorcycle-Seat-Cov...
You might be able to modify the existing pan using the heat gun method, but it would need to be a GOOD quality heat gun.

Or if you really want that banana seat look, just buy a used Honda seat and work from there?