lwien, regarding your tip:
I did the same thing (removed the retaining clips) a couple months back and couldn't believe the difference either...until I realized it was doing some damage in my circumstance.
A quick preface: while I have never consulted a medical professional regarding this (aside from quick chit-chat with a nurse friend) I'm pretty sure that I'm unfortunate enough to have some form of allergy to MJ. Fortunately the vapor (and smoke when I combusted) itself is completely welcome in my body, the issue for me is skin contact: grinding or handling herb can often result in a small allergic reaction for me (hives and sneezing fits, well in line with the experience I have with many other pollens). Thankfully my usage is planned around the issue and I don't have to handle herb explicitly often.
In any case after removing the clips I enjoyed the extra air-flow greatly, but I realized quickly that sucking on the stem to fill it or hitting the PD directly (w/o water filtration, something I rarely do now) meant that a lot of small particulate was getting through the stem. Now, having said that I have an allergy or sorts to the product I know the following wouldn't be true for everyone, but when sucking on the stems without retaining clips in place I would get an awful irritation in my throat and lungs (almost an itchiness always accompanied by coughing). Initially I wasn't sure what was happening until I realized that I was also able to feel the particulate (a dusty, grittiness) in my mouth as well.
In either case now I keep one retaining clip in and leave the exterior one out...this seems to solve the issue for me. Also, I refrain from using the 'straw method' to fill up the stem whenever possible and just stab into the grinder/dish/etc. with the stem. Finally, I almost always use a bong/bubbler as it seems a logical (and actually makes a noticeable difference of course) defense barrier between the PD and my airway.
I bet I wouldn't have a problem if I didn't suck on the stems at all but I put one retaining clip back in just to be safer for me. Just my
and experience.