I can't get even close to locking front wheel with road hydraulic brakes which is a shame in my opinion. There is a ton of grip on soft, slick tires when road is dry and clean (most typical conditions), way more than on mtb (damp forest, sand, loose rocks, etc).
Only reason I see, why road brakes are so pathetic is that some guys (and girls) don't want or can't handle brakes that are able to throw them out of a bike. Also very strong brakes could be problematic in rain.
This sentiment sounds contrary to most I've read about road disc brakes, and certainly doesn't agree with my own personal experience, albeit only with mechanical (Avid BB7) and hybrid mechanical (TRP HY/RD) disc brakes. It sure sounds like something is off with the brakes on the bikes you've ridden if you feel like there is no chance of locking the front wheel.
This past Friday I went out on my bike with the front-only BB7 disc brake and had no trouble stopping on some slight inclines even with a ~55 kg. trailer load attached to the bike (Burley trailer plus two kids plus a bike and misc. kid stuff). I have a 160mm rotor on that bike.
For using a 140mm rotor up front, for someone on the lighter side, I'd say go for it. My full-disc road bike with the HY/RD calipers has 160 front/140 rear rotors and even using just the rear provides surprising stopping power. If I was going full weight weenie on that build I'd have gone with 140mm front no question. As it were, I wasn't all that focused on lightweight and I was ~15 kg. heavier when I built it, too. Down to 67 kg. now and the 160mm front has gone from more than adequate to serious overkill.