Just copied the new native pdf job on the kindle and tested it. Globally smaller than the pdf's made with your multitasking suggestion, and text is top quality, on par with the native ebooks quality, plus page turning is as fast the smaller, splitted pixelated pdf's... images are clear and better... if only I started reading the book in this quality instead of picking up just now
Apparently pdfsam made somehow the smaller joined pdf's slower to read on the kindle, and since I couldn't succesfully run jpdftweak on those I wouldn't know if that's better.