The (Frontier) 85 requires the top tether (in harness mode) after 65 lbs. It's encouraged at ALL weights, but it's mandatory after 65 lbs. The SICT encourages the TT at all weights, but it is not mandatory at any weight.
Some vehicles have a stated LATCH weight limit, which, includes the top tether anchor (TTA). For instance, in a Honda, it's 40 lbs. In a Chrysler, it's 48 lbs. Toyota, for instance, defers entirely to the child seat for weight limits.
Prior to 2011, Ford had a lower anchor weight limit (48 lbs) and a separate top tether anchor weight limit (60 lbs, IIRC). In 2011, Ford changed their stance, keeping the lower anchor weight limit at 48 lbs and deferring top tether anchor weight limits to the child seat. Confusing stuff, huh? Looks like you're on the right track, though.
Unless your child is exceptionally heavy for her age, she will likely outgrow the harness on the Frontier (either version) well before she hits 80/85 lbs. My DD is 7 and a half years old, 49.5" tall, and 60 lbs. She's, I think, in the 80-ish something percentile for weight and she's got about 1/2 (maybe less) left to grow on the last harness slot of her 85. She's average height with a generous torso. It's top tethered, but she'll definitely have outgrown the harness by the time she reaches 65 lbs.
Additionally, installing a Frontier using the long belt path assists with head excursion (due to the shoulder belt routing/placement) reduction. So, if you have an untethered seat, there's still some potential benefit with the shoulder belt playing along.