I would just implement it and go measure, and be ready to roll back. Pound it flat with artificial traffic at 4am?
(I am one of the authors of fq_codel and cake, but I do not have enough data either, on how well this stuff scales on given bits of mikrotik hardware). In most cases it is the per customer shaper that dominates the cpu by a factor of 9, and the underlying queue type be it a fifo, sfq, fq_codel or cake, adds only a tiny bit (cake is about 2.5 times as slow as fq_codel but does more, and again, it is the shaping cost that dominates). If you are low on memory, for folk running at less than a gbit, you can use memlimit 8mbyte rather than the default 32Mbyte.
On x86 gear we have individual implementations of cake scaling to 10Gbit/core. On 500mhz single core mips, cake scales to only about 80Mbit. your mileage will be somewhere between those. 
See also.
http://forum.mikrotik.com/t/some-quick-comments-on-configuring-cake/152505/1