Check it out...
I agree, loading the filter with more spice, a lot of it will travel down the neck (if you are using a 'machine' type of device), and chances are you will only actually inhale half of this dose...Its all good, usually that is enough for yours truly, but here it where it gets interesting..
After time, say 6 or 7 uses, your filter will be automatically charged with unfinished spice from before. This, upon being heated, is smoked along with your initial dose, to create a HUGE cloud of smoke, and a helluva journey..(this happened to me earlier today)...You can always heat it up from the outside with a lighter, tilt the machine upside down, and let all the brown dmt goo slide back into the filter, where it wil be vaporized on contact...
Now usually leftover spice seems to be okay, providing some of the best journeys I've ever had...In my experience,Its when its REALLY old, say, a couple of months, when it begins to affect the experience, and then you'd have to clean out the pipe, or at least change out the brillo pad.
When time comes to actually clean out the goo from the pipe, I'd try a Re-X tek or something with acetone, to retrieve as much product as possible.
Hope this helps some!
"The search for Truth is the Greatest, if not, most Sensible form of Rebellion."