Well...
using the tyre calculator:
0.46%
or 0.34 MPH at 70.
And 10 mm thinner too.
You could put two of them on the front without offending anybody, apart from the style Police.
If you put them on the back it might get interesting on the limit cornering.
You aughtn't mix 165 and 155 on the same "axle" as the insurers start to disown you if they find things like that, after the event.
Also, take care with bits from the scrapper:
if it died at the MOT - probably OK
if it rolled there may be internal damage you can't see.
Look at the car they came off to decide.