Well I am not completely impressed with the Leo as of right now. I have had it now for little over a week and I have tried vaping different juices in it. None of which have given a very good TH. I used the same exact juice in my M401 and my dse 901 with a LR atty, and the TH is good to strong. Now with all the praise that I have read about the Leo being good TH, having good flavor production and Lots of vapor. Only 1 has been consistant, the vapor production. I Am also getting frustrated with the carts they seem to not get the juice down to the Atty. I have tried several methods to get the juice to flow. Tried the BFM, the plug mod, and even purchased some poly fill and cut it just a little longer than the cart. None of these are working very good to get the juice to flow. I also have had issues were I filled a cart 3/4 full and placed in the poly fill, then topped it off and took a few puffs and have had a gurglling sound. Removed the cart and it was like half of the juice i put in the cart has flowed into the atty. I have used PG, VG and PG/VG mix. At times the atty gives off a burnt taste with different juices, then other times good flavor and lots of vapor but no TH. Then a few puffs later the flavor is gone and only the vapor is strong. I have used 11mg, 18mg and 24mg. Usually I vape with 18mg, and the TH is satisfying on my DSE 901 and Ok in my M401. The Leo is just not giving me the TH I expected. I do however like the battery life I do not have to carry around 5 batteries to keep me going through the day if I am out and about.
IMHO I would have to rate the Leo at 2 out of 5. I am trying very hard to like this thing, but it is almost a hassle. I am going to keep at it because I really want to enjoy this thing, but just not getting that wow from it.
What you're describing is a problem with the old style carts, designed for the big battery mods, in general.
I have screwed around with just about every 510 cart mod, and sometimes they work well, and then they become crap. Too much of a hassle. Messy, inconvenient, sloppy, too much trial and error, and when you do find a good one, the next day it's often worse than when you started.
Carts just suck.
That said, the 801 cart style works better than the 510 (which is a terrible style for carts, mega possibly aside). I would bet money, that if you had an ego style 510, you would be far more frustrated with the quality of cart vaping.
See the thread I posted here. There is light at the end of a hopefully short tunnel. An LR atty, to drip with, for maximum flavor, and a tank mod, for maximum convenience and better wicking, are both on the table. I certainly wouldn't go selling your Leo.
If anything, grab an adapter from avidvapor.com, and run 510's, or 306's, or even stack a 510 to 801 stealth on top (I can go test the fit, to double check if that works), and drip on that, before you ditch out on the whole program. 801 LR dripping will blow away your lack of flavor issue. It will blow away your 901 as well. You'll seek that forever, after, if you're like me, and flavor is priority #1.
Basically, the quality of the vape is atomizer style + method of juice getting there + personal preference = happy.
In the option of juice getting there, the progression is: Dripping > Bottom feeder > Cartomizer > Cartridge (modified by the overall wattage, to personal taste).
If you're not happy with the Leo, it's almost certainly the fact that normal cartridges just suck overall, and behind that fact, that there's no low resistance option yet.
Love this board for all the great info shared. 
