I had a chance to compare a Megane RS 275 with my R. On the autobahn, acceleration-wise, the R pulled away. But this was never a strong side of all those Meganes. You need to up the power to 300ish on the Megane to be able to keep up with the RCZ R or sloooowly walk away.
Then on the more important roads, B roads, the Megane shines like a supernova. Already the second generation of Megane, from 2007 upwards, and foremost with the special edition of the Megane R26, they became ballistic on B roads. Sensational feel for the steering and balance of the car. According to the magazine EVO, the Megane R26 was the first ever car with electric assisted steering which transmitted propper road feel into the steering. I wholeheartedly agree with that. If you had the skills, you could give a run a Lotus Exige on B roads with that Megane. Yes, it was a bit feeble with the 230 horses. But they could be upped with stage 1 map to around 250 and more torque which was then absolutely fine and sufficient for this chassis.
Then the next generation, and especially the 1st facelift and the the 275 version, upped the game dramatically. If you know what car feel is all about, you stepped into it and after only 200 meters of driving you knew it was a serious driver's car. Such a fantastic chassis, suspension and this chirurgical steering. Again, the only small downside was again the engine. It was much better then the original Megane 250 and its predecessor. But never a really engaging engine. A stupid Golf GTI had a better drivetrain than the Megane, feelwise.
And this is where the RCZ R stood out. The turbo engine on the R is just a piece of metal to adore. It revs like a NA engine, it pulls up to the rev limiter, it is all alive and adds a huge element to the fun factor of the R. Nothing to compare with the 156 or 200. Just another league.
Yes, the chassis and suspension is great too. But I reckon the latest facelift of Megane, the very last stance with the 275 version, is a bit better. It rewards you a bit more with fun and emotions when being pushed to the limit.
You can do silly things with the R too and have a plenty of fun. Don't get me wrong. I've been there. But the Megane is like a tiny tiny bit better. But then you think about the engine and I would go for the RCZ R any day.