Weight: 390g .... plus the 398g of the Switch + Joy-Cons itself = 788g
No way that such a clunky brick is comfortable in handheld-mode, a (much cheaper) wired external power bank in your pocket or on the table will be much more comfortable.
Also you can forget the estimated release date of that accessory... which kickstarter or indiegogo estimate hasn't been delayed for at least 6 months or even several years?