I'll give this a shot:

Imagine that the miner you bought is a *dollar printing machine*, which you buy with *dollars*.
When trying to figure out if it makes sense to buy this gizmo, do you need to know how *the dollar* price will change relative to the yen?

Hopefully, you've answered "No.  All i need to do is know how many dollars this gizmo will print in its lifetime, and how much i have to pay for it.  If I pay less than it makes, I win.  If not, then no deal."

Now substitute BTC for dollar.
Potip:  If you think bitcoin's going up in price, buy bitcoin with the money you would have spent on the mining gear FTW.

You should leave the conversion % increase at 0 because if the mining equipment isn't going to ROI in BTC terms then what is the point of going through the trouble of mining when you could have just bought BTC.

Sierra is a nice piece of hardware from current point of view.
However I'd say it depends on the point in time of final delivery, what KnC offers begin of next year and how cointerra does..

Jupiter customers from day1/2 have until today ~50% of their investment back in pockets (from fiat view point, only).
So it might work out if you are one of the early birds taking a high risk, but for later ones the margin gets smaller and smaller.

Keep in mind a 'money printing' machine attracts greedy ones from all sides, stupid ones as well - I saw on ebay mid of September 7 bids for old avalon 160GH/s, $11.9xx

The Jupiter is sold by KnCminer not Hashfast.  It is $5000 for November delivery. It will not ROI in its lifetime.
If you buy a mining device you have to do your homework, pick the right company and make sure you order on day 1 to secure "first day shipping." Then cross your fingers and hope they deliver on time.