Regardless of the compression level, the audio data remains absolutely the same, anyway it is reconverted to raw audio data during playback. If you really want to check, compress at level 1, 5, 8 and then reconvert to wav and compare the resulting audio data: absolutely identical!
As to how FLAC achieves different levels of compression while maintaining the same data?The gist of it is that compression works by finding the redundant data within a file and exploiting this redundancy to write the exact same information with fewer words, now there are different algorithms for finding redundant information, the more effective ones are also more calculation-heavy, trading file space for heavier CPU load, that's how different level of compression exist. But in the end, FLAC is lossles, with exactly mean that the information is strictly the same for the compressed file (regardless of the compression level) and the original file.
Finally with modern CPUs, level 8 compression is far far from being to CPU intensive to use.