With a nominal 3m^2/300km, and 0.025m^2 I get a detection range of 90km (range and cross section both raised to power ^1/4).

(Nominal max range if ambiguity not an issue for B-52 class "barn door" is 720km).

With the 6m^2/300km baseline, I get 75km and 605km. There could well be additional factors not considered, but 1/10th seems massively optimistic for this factor at first blush (in favour of stealth).