On ARM it is no contest, at any size.
Funny little $.10 PICcy things may not have a C++ compiler available. Then, the C compiler is better than asm, if you can fit. Otherwise, no excuse.
I don't think C is always a win, but to say to never use for new projects seems overly biased. Why is your opinion about C so strong? I am curious.
But a C compiler restricts you to the C subset. Many of the most useful features of C++ generate no extra instructions, but make the code more maintainable. There are libraries that compile to no code except exactly what you call, optimized down to exactly the circumstances of the call, wholly inexpressible in C, that you would have to open-code directly in C, gaining no benefit from the maturity of a library.
In other words, in the absolute worst case, you write the program using only C features, and get the same program. But there is never a reason to opt for the absolute worst case when you have a better choice.
So, no, there is never a reason to confine yourself to the C subset, in a new program. "for (auto e : v)" is universally better than "for (i = 0; i != N; ++i)". Zero cost, better code.
But I would never, ever suggest using an IDE, under any circumstances, for any language.