The streaming FFT demo does not re-scale the outputs of the FFT -- the scaling is set to 1/2N, so the output will be 2048 times smaller for a 1024 pt FFT.
The IFFT results then have quantization effects on the output sine wave as well as being scaled relative to the input sine wave. The output waveform will seem to be distorted, which may cause users to be suspicious about the quality of the transform.
To avoid this issue, change the FFT and IFFT scaling to 1/N (1/1024), and use reinterpret blocks to re-scale the outputs of the FFT before applying them to the IFFT.
A reinterpret block with the output binary point set to 5 will left-shift the decimal by 10 digits to compensate for 1/N scaling. The output waveforms from the IFFT will then be smooth and resemble the input waveforms.