Does Prius III has anti-roll bars?

yes, twist-beam design works as anti-roll bar without one.

however in my humble opinion Gen III will benefit from body roll reduction on mountain/twisty country roads. It seems to be doing fine to resist initial rolling at moderate loads but then falls into turn heavily if pushed.

With respect to twist beam in some instances twist beam axles are designed with anti-roll bar which is installed inside beam. In this case the anti-roll bar is ~2/3 of the wheel base, looks like a straight bar with mounts at the end and adjusts the stiffness of the beam itself.

The TRD/Hochkiss is a full bar attached to shock mounts was probably designed as such to resistance more evenly then the axle does, and reduce body flex/stress at mounting points.

EDIT:
below the example of anti-roll bar on twist beam:

 

The strut tower bar helps roll quite a bit as well and is very easy to install.

I am very impressed with the way my car handles now and it is on worn out yokos, going to get new tires tomorrow.

 

See my LRR tire thread in the Economy section.

 

So after a couple futile attempts to get Eibach anti-roll kit (apparently discontinued), my TRD anti-roll bar and Tanabe STB/lower brace coming tomorrow/friday. I know what I'd be doing this weekend

BTW for those looking for rear anti-roll bar part #s, here is the list:
EIB-8290-312: Eibach Anti Roll Kit (discontinued) ~150-160$ shipped
TRD PTR11-52091 ~240$ (with links)
TRD PTR11-52080 ~150$ (w/o links discontinued due to breakage but still available)
TRD PTR11-52090 ??$ (replacement for PTR11-52080)

 

Progress update: Installed anti-sway kit.and WoW!
Part of my commute route takes me over some narrow windy country roads, off-camber turns, crossing ravines, etc.. some of those must have been indian foot trails 400 years ago. Prius no longer falls into turns as it used to, no body roll to worry about. This gives more control and steering precision when pushed closer to limit. Still understeers, much less and in more civilized manner. No oversteer to worry about, Michelin Energy Savers will break loose and start drifting way before any wheel lift.

Have to wait 'till next weekend to put STB.

 

I installed a TRD PTR11-52091 ~240$ (with links) on my 2011 last Saturday. I have not had a chance to put it through some good corners yet but I can already feel a difference. Next mod will be a STB and then some more chassis bracing under car if necessary. Between the Eibach lowering springs and the TRD swaybar, the car is starting to handle like I want it to. It no longer feels like I am floating on a cloud of pudding while moving.

 

Nice! It doesn't even feel like a Prius anymore does it?

 

I drove for ~week with swaybar then put Tanabe STB. With STB the difference was subtle; a little bit more grip from front, a little bit less steering effort and sharper steering feel. THe Tanabe lower brace is box waiting for oil change next week. As is it doesn't feel the lower brace will make any difference, but I could be wrong. If you wanna wait I'll post review in 2 weeks.

The STB was 107$ and lower brace 88$ +20$ shipping from CO.

 

I installed the TRD rear sway bar a couple of weeks ago. While it's no autocross car, the improvement in handling is delightful. The bar should be standard equipment.

 

Nope. I agree, rear sway bar should be standard equipment. Mpg on this tank is going to be a bit lower as it is harder not to push the car a little more.

 

Please report on the brace once you get a chance. Where did you buy the STB at that price?

 

TANABE FRONT SUSTEC STRUT BAR: PRIUS 2010 - Auto Parts
TANABE SUSTEC UNDER BRACE: PRIUS 2010 (FRONT) - Auto Parts

 

O'k got lower brace in and the first impressions:
- it is definitely doing something; it stiffen up front judging by the amount of vibration coming through the steering wheel.
- it seems that front grips a little bit better, by how much hard to say haven't had opportunity to push it hard enough yet.

Installation was a breeze, put it in while changing oil. About 5min, you just need a long 19mm socket or short extender. Out of negatives it reduces clearance by ~1.5".

Overall comparing to previous TRD swaybar + Tanabe STB the lower brace improvement is very small if any at all. Perhaps the law of diminishing effect or placebo? Not sure if the brace even worth it as with swaybar+STB the 15" LRR tires would loose grip before suspension/chassis becomes an issue. And there is a clearance issue. On other hand it is only 90$, and if it improves grip even a little bit..

Disclaimer: yes comparing to overinflated LRR tires the grippier 17" wheels would make big improvement, but that would also meat that MPG will drop into 40s from current 59-60MPG. Not worth it IMHO Prius is primarily a high MPG not performance car.