Maybe it would be possible to design the scientifical computational tasks so that they are very similar in terms of stress for the CPU, but not completely even. You could then state that the benchmark offers comparable results with a statistical error of something like 2% +-0.5. You can also average the error by running the benchmark multiple times before you compare the score with other devices.
Is Mersenne prime verification really scientifically useful? At this point, getting one more number verified seems about as useful as stamp collecting.
Mersenne prime verification is useful for the science field of mathematics, where the results can be used to (in)validate theories and predictions of (Mersenne) Prime Numbers. Sure, it is a niche with little contact area with the rest of thr world, but so were Elliptic Curves (used in cryptography), Set Theory and Relational Algebra (both heavily used in relational DB technology).
I don't see why we should be limited to Mersenne prime verification, there are many other scientific computational projects in various fields that could benefit from it, astrophysics, molecular biology, genetics, chemistry, climate study, cancer research, ...
