I've got almost 7 gals out of my 4.6f , how much more can you ask for, its still does wheelies on command and i expect to get 10 gals out of it, as for the 5.9, i'll just play it by ear. nither one are speed demons, but they were never expected to be, 20ft + jumps
best advice for the 5.9 is to ditch the stock pipe.
go with ether the cen genises stocker or the ofna 086, both or superior to the stock 5.9 pipe.witch imo starves the 5.9 for fuel at top end.
switch that pipe out and you'll get better proformance and run at cooler temps. you may also want to watch you midrange adjustment, alot of ppl have had problems withe it ether backing off or just plain falling out,most are putting a small amount of rtv on them. peace