Sennheiser MX-W1 is a wireless Ear bud.
whereas
Sleek Audio SA6 (in wireless mode) is a wireless iem.
Its kind of hard to compare between the two with regards to fit as both are different.
However, do note that the mx-w1 implements senheiser's very own twist to fit system. I tried their sennheiser mx-85 and i seriously didn't like their twist to fit system and it is always hurting my ears so much, sold it.
SA6's isolation, with their stock dual flange, is *very* isolating IMO. Compared to other iems using silicon flanges, SA6's iem is relatively higher by a few notch. And, this is called "isolation", and not "Noise canceling" like some of sony iem.
However, it is possible to get custom moulds for earbuds to turn your mx-w1 into a wireless custom, but, including the cost of the mx-w1, it is far more costly than sa6 wireless.
With regards to amping, the improvement is obvious when i am pairing it with the RSA Predator. I sold my fiio e5 before I got my sa6, so i can't comment on that.
SA6's pros is that it is a very balance iem, which I feel is very suitable for majority of the genre of music.
My 2 cents.