All the stalking I've ever done has been at red hinds on the hill, and I have found the 120gr BTs loaded by Norma work very well with chest shots. Some might dislike the risk of occasional meat-trauma, but I've never had a deer go far after being hit, and you can't make an omlette without breaking eggs.
Are they really cheaper than the RWS 140gr DKs? Last time I bought them, I think they cost the same, or perhaps a little more.
There are some old threads on 6.5x55 factory loads which might be worth searching for.
My summarised experience:
1. American loads tend to be rather slow
2. Norma Nosler Partition 140gr only make 2550fps from my rifle, and whether for that reason or because the bullet it just too solid for the quarry, expansion on hill hinds was not very good.
3. My 21-ish" barreled Finnlight 75 gives markedly lower velocities than those quoted on the packets (usually tested with 24" barrel)
Of the stuff I've tried
Remington, Federal, S&B and Privi gave inadequate accuracy in my rifle.
RWS 127gr I found too slow for the bullet weight
Norma 140gr didn't open enough for my liking
I therefore use Norma 120gr, and RWS 140gr DK where BTs are unwelcome.