Short question, long answer!
I wrote this "Diana mans" barrel before:
http://wwaccuracy.com/showthread.php?t=6149&highlight=newbie
We bought dozen Broughton barrels together to our Team and every Team member got his barrel before we slugged those.
We slugged those Broughtons together, there were slugging gunsmiths Tomi Korpi, Tapani Nousiainen,Juha Söderholm & 5 other RFBR shooters.
This barrel feels best or as good as second one of those dozen barrel.And this barrel was coming to me.I didnt make this project ready and I sold this barrel to my 3-P shooting friend and gunsmith Jan Stenroos put it to Diana action.
In one ammo patch test it gives 4 x 10 shots groups average 10,5 mm (0,41 ") outside to outside measuring and pair RFBR shooters said to me that why you sell this barrel to position shooter.
My 3P shooting friend has many 3P rf and he likes more other ones (stock & trigger reasons) and decited to sell those Broughton&Diana irons.
They were little too heavy to LV class RFBR shooting and Mr Tapani Nousiainen shorted action & barrel to this RFBR newbie and bedded irons to straight lined BR stock.
(Ps.All those Diana RF.s coming from East Germany long ago original barrels were fastened to action with "tin",maybe silver. (no thread)..One now retired top gunsmith change many of those with "screw thread" and blueprinted those same time.)
Before I answered you question when RFBR shooting happens inside range to 50 m RF is maybe 80 % & shooter 20 %.Outside shooting to 50 m on very bad condititions maybe then RF is 50 % and shooter 50 %.
If shooting 100 m with RF outside on very bad conditions then shooter is maybe 90 %....
Sometimes BR shooter find "the right/best condition" outside and can win with not so good RF.Nobody finds this always!!!!
Your question: Do you believe the Diana would seriously challenge the Turbo here in the USA.
Sometimes, in some "shooter condititions" YES, constantly NO!
Ps2."Turbo men" have won most of RFBR competitions to 50 m here last years. Mr Tomi Korpi & son was top with his Falcon on its best times.
Tomi&son has been on line very few times on year 2021.
To 100 m it has been different story.
BR, Timo