I have refilled Blu carts but it really seems not worth it after its finally done. Dripping seems to work better than refilling carts although its not very safe to drip while driving lol. Expect to fly through carts just as fast as you are now and still only use up 1/3 of the liquid in it.
If you use the PTB mod you'll find you're using pretty much all the liquid, and your vapor production should increase and become more consistent, too. The PET wicks the liquid faster, more consistently, and more completely.
I'd be interested in the Sidesho if it were smaller and if the PCC were as useful as blu's, but not so much.