I would expect the interior volume of the 9" Donut and the HD3 to be very similar so clearing them would be similar depending on the MP one is using. The mouthpieces I use are pretty thick (mostly 16mm OD and 12mm ID) so I have no issues clearing any of these 3 pieces. YMMV.
I don't think clearing is similar, because of the form factor difference, perc orientation... Good point about the mp, I have Goo and Phineas ones I use with my TAG through 45° angle adapters, diameter is different but so is length and other aspects...
A piece that is particularly difficult to clear, is the large bent neck gridded inline from Biao that is 14inch tall with a very wide can, like a large bong, so I think most of these rig designed pieces even though they are large are easier to clear by default comparatively.
I have always wondered where the TAG stuff came from, but their prices are relatively low for US vendors, but high for China.
Yeah, they are expensive on DHgate, the ones that have been found, but still cheaper than TAG prices, for the most part it's hard to find the exact same piece... Yingmin5 Lumin88 actually have quite a few similar pieces, and yes very thick glass, same styles, with slate variations, makes me think Genesis Glass could have an exclusive line with TAG, or it is another glass manufacturer (or multiple) it is similar to GRAV in that they have some exclusive China suppliers with exclusive pieces they are not allowed to sell to anyone else, they curate the designs and quality control for you, and mark up price!