probably has to do with these things (and maybe more):
1) length of the head (longer)
2) grain distribution over length (spread out)
3) your accustomed stroke
The Teeny style is only 24ft long in the head and very densely packed, a shooting head among shooting heads.
Full sinking lines are thin diameter long headed fishing lines that are best cast with a long even keeled stroke and more line in the air (in standard WFS configs).
If you took the same grains spread over 24ft and stretched it out over 50, the line will feel very light until you get a lot in the air.
I would venture that your casting stroke is also tuned into the shooting heads, which are great "get down to business" lines and catch a lot of fish.
BTW - you don't want to open the loop to try to get a better load. If anything the loss of load could be largely due to a circular stroke opening the loop up and dissipating the energy (not focused).
Also 325-350 grains over 24-28ft loads the rod well (I fish it all the time). A full sink of 300 grains is probably that over a lot longer head 35-45ft (?) so it's probably feeling a little light until you get a good length out there.
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