You should 100% do it. There is a little bit of downtime between the shipping, but you said that is all the cost you take and its a hassle for several days, but that's it. After that you have a better graphic card which performs in many titles 15% faster, in fact when overclocked most 1070ti match the 1080 FE. That is a big deal since the 1080 FE is on average 25% faster than reference 1070, obviously custom cards are faster and cooler so its different, but in essence you are seeing a potentially 1080 like performance if you overclock.
The biggest downside is that the 1070ti does indeed use as much power as the 1080, when overclocking more than the 1080. But apart from that there is no downside!