Newtons still seem popular. They are stiff enough, not the stiffest but I've personally come to appreciate it because a bar with some give is less fatigue inducing. If you want stiffest, look for FSA Energy Traditional. Very high S/W ratio.
In my view the Newtons had their day because the top end to the drops is less sharply curved than any given modern bar that the bar feels a little awkward (maybe uncomfortable) at the extreme ends of the tops.
Most modern bars are sharply turned towards the ends of the tops which results in wider clearance at the tops.
ZIPP Service Course and SL in TB are very nice bars. TB yet still very modern features such as compactness and an open feel. Not as stiff as FSA Energy but slightly stiffer than Newtons.
There is alot more feedback I can give if you wish to know more in detail.
I don't believe there's any solid reason to give up the Newtons for any other alloy bar. Great bar. Carbon well, 50/50.
Post some deflection numbers to back up your claims.
The Zipps are not compact. Shallow(er) drop than some traditional bars, but at 88mm reach and a measured 130mm drop they are anything but compact.
I've had Rotundos, Zipps, Dedas, TKOs, NeoClassics, and the Specialized bars. I rode the Rotundos for a long time and loved them and have been on the Zipps for a while and they seem great as well. Dedas had a weird ramp to the hood, TKO drops were uncomfortable in my hand, and the NeoClassics didn't have enough ramp to the hood. I have yet to try the FSAs as they have a bit too much reach and drop for what I need.
Don't take me too seriously. GramzStrava