It never becomes natural for me. Not even back on the OXBOX F1 game that I mostly if not solely played manual. In this game it would only be worth it if I were concerned with competing with other people.
I noticed a huge improvement in times going to Manual or Manual/Clutch. With some high-end race cars I’ve found Manual is better to avoid damage and it shifts almost as fast. With slower vehicles I always use Manual/Clutch. Tweaking the dead zones for the buttons has minimized Sim damage and is faster.
The benefit is more than gaining time on straights or out of curves, I found you can control speed going into curves if you time your downshifts right.
A good example of the importance of Manual and going above the red line is the soon to end co-op Seasonal Championship for the Santa suit against Unbeatable drivatars.
If I use automatic I can’t beat all the drivatars (which I tried for the fun of it) but with Manual I’m #1 if my teammates don’t wreck me. I built a Unimog tune with the stock transmission which has 10 gears. I quickly realized that to beat the drivatars I had to exceed redline substantially and it’s working great. The 3rd race it’s hard to beat the Iron Knight in the Unimog unless I’m near perfect driving in the tight uphill section by staying high in the power band with frequent up/down shifts and using the AWD to my advantage because they can pass me on the long straights.
NOTE: The Iron Knight is a better choice if you are smooth on the throttle and don’t spin the rear wheels, but I liked the challenge of using the Unimog.