Well, Mosconi run his 526 in a 8-footer with loose pockets (or that's what they say..)
I'd say that running 250 in a 9'' Brunswick Tournament Edition (Metro or GC V) is harder than running 526 in a 8'' with buckets. The comparison would be otherwise much more than (app.) 1:2, but it must be true that after couple of hundreds, no matter how easy the table, it must be exhausting and needs lots of stamina and focus.
In anyway counting the records is somewhat useless (or at least unofficial), unless the standards of the table is measured/settled.
In another comparison I could say, that running 100 with the Brunswick with Tournament Edition pockets must be about as hard as running 250 with the ordinary Brunswick with buckets.
True, it must be exhausting to run so many balls, Earl Strickland once ran 408 in practice and he say it was the most exhausting thing he ever did on a pool table.
I don't know how loose the pockets were on the 4 x 8 table he ran the 526 on but Mosconi certainly could run big numbers on tight tables. My uncle saw him run something like 240 balls on a very tight 9 footer in an exhibition, sometime in the early 1960's. The amazing thing is that he only used the front 4 pockets for the entire run! Not one ball went up table. mosconi also had the most 125 and out runs on the old 5 x 10 tables and some of those had brutally tough pockets.