Top end Kr808D-1 models are regulated to 3.6v. An ego is 3.2v. You actually get a better performance (just from the battery's output perspective) out of a kr808d-1 than you do for an ego. Performance is subjective though, as you get a longer run time out of one ego battery than you do compared to one Kr808d-1 and you also have a greater ohm range you can use with an ego. You shouldn't go lower than 2.5ohms on a slim ecig, it's recommended to use batteries rated 650mah or over for low resistance heating elements.
When it comes to a Kr808d-1 vs any other 3.7v mod or ecig, it depends on the user of whether form factor is more important or if they're ok with 2.5ohm cartomisers or would prefer to use lower resistance and you're not ok to charge slims more often. Though, 420mah "slim" ecigs exist. I have a pair of 320mAh volt batteries which I prefer to take over one ego battery as they're slimmer and I get the same sort of run time between them at a higher voltage, with two flavours to rotate.
I have a few Kr808d-1's and a few slim 510s. The kr808d-1 performs better than the 510 as it's 3.6v instead of 3.2v with a higher mah rating so there's a bit of a better hit and runtime and I'd say the 808 cartos are better than the 510 ones (personal preference).
So really, you can get a better or equal performance out of a slim ecig but not if you want to go low resistance vaping. If you're happy with the kr808d-1, then that's cool too

you've got a higher voltage unit than an ego.