Some months ago, after reading posts on the kia forum about the manual requiring excessive amounts of pedal effort to modulate the clutch, I had to test drive one for myself.
I wasn't able to find an optima close enough to drive, but did locate a sonata 6spd. Given the similar nature of the two cars, I didn't think there was any difference in the driveline of the two.
After my brief (20 min?) test drive, the only conclusion I can make is the folks making the above claim must be used to driving civics or vw's that require minimal effort to operate. The pedal effort didn't seem drastically different than most other none sport car cars I've driven. The uptake felt normal for a non sports car clutch. The friction zone was larger than I what I was used to. With both my camaro and now the G, the clutch doesn't start grabbing until the last 75 or 80% off the floor. With regard to effort, the camaro in its stock form took LOTS of effort to depress. After driving that car for 15 years I was very used to the stiff clutch pedal and actually preferred it.
On the G, most owners end up either removing the clutch pedal assist spring altogether, or replacing it with a much weaker one. I opted with the second choice. Otherwise, the pedal is free to wobble with the small amount of free play present at the top of the pedal stroke (clutch completely engaged). The result is much better clutch feel/feedback. With the stock spring, it's very hard to feel when the clutch is actually starting to grab. Engagement is evident only by the car beginning to move. Also a good way to burn up the clutch if you're on an incline. With the weaker spring, the pedal feel is no longer linear, but progressive. As the pedal is released higher off the floor, the level of resistance increases. For comparison, the stock spring was rated at around 330 lb/in, the replacement, 50 lb/in. With both hands, I could barely make a deflection in the stock spring, with the latter, I can fully compress it. Here's a pic http://img189.imageshack.us/img189/8373/p1020850z.jpg
I'm guessing this clutch delay valve referenced above serves a similar function, but is placed in the hydraulic portion of the clutch system rather than the mechanical pedal. During my test drive I had no issues with stalling the car or taking off on inclines. There was a sensation of lack of feedback, and the shifter itself felt sort of rubbery - as though the shift gates were not distinctly defined.
I think it is important to have the seat in the right position so that you can fully reach both pedals and have good control when modulating the clutch. I find playing with the angle of the lower cushion can help. Perhaps it wouldn't hurt to look under the dash to see just what the clutch pedal arrangement looks like.
This is what it looks like on the g37/370z
Another view with the spring removed.
If you're still stalling, try to figure out why and compensate. If the off idle response isn't there, perhaps there are some stops that can be adjusted so the throttle doesn't need to get push as much to initiate a rpm increase.