I own both M800 with 18k M nib and Conklin all American with steel M nib and they do feel different.
More than a real difference between EU and US nibs, you should consider that on average modern Pelikans have slightly wider nibs (wider also vs EU pens).
The other point you need to consider is that Pelikan pens have rather generous flow.
Then do not forget you are comparing a 18k nib with a steel nib.
Especially the first two things together suggest that if you are absolutely unwilling to experience a thinner line with more restrained flow, you might be better off with a B nib on the Conklin.
In any case, don't expect exact same behaviour, when comparing a different pen and a different nib and a different filling system, as you are bound to find different performance.
All in all, the M800 is no doubt in another league vs the Conklin, which is however a nice workhorse pen.
Edited by sansenri, 31 May 2020 - 22:24.