Which microcontroller for a program with many floating point operations
Hm, until you do a bit of benchmarking to show that it's really the FPU, I'd heavily doubt this is about the Cortex-M7F FPU being slower (it's really not, never seen that).
Generally, try to make sure you're not inadvertedly doing something like soft-Floating Point math (-mfloat-abi=soft), or aren't using math libraries that have been optimized for the STM32F4, but not the F7. Make sure you're compiling for ARMv7-M or ARMv7EM.
The fact that you're putting a processing rate to this: This sounds like a DSP workload. So, make sure you're really using the DSP instructions: both the M4 and the M7 should have single-cycle Multiply-accumulates, so a 200 MHz M7 should in any case be twice as fast as a 100 MHz M4 if these are used. Your compiler should infer these, but sometimes a bit of hand-assembly pays.
So, either you're using a compiler too old or set to not use the FPU, DSP instructions sensibly, or something else is going on here.

From a general DSP engineering perspective: often, there's much to solve in algorithmically or programming inefficiencies before specific properties of FPUs become relevant for application performance. Since already your 100 MHz Cortex-M4F is a pretty strong processor, 15 kS/s of throughput does sound like a pretty hefty DSP workload (700 CPU cycles per sample!) and it might really make sense to ask a question on the DSP StackExchange sister site, describing the algorithm you're doing, and specifically about how to do it.