But I have fanless video cards with lower TDPs and they can get quite hot.
True, but the TDP itself then has to be matched by the thermal conductivity of the cooler. An entry level passive card might hit 70C on only 30W but it doesn't mean that a 120W will go any higher, you need to know the thermal conductivity to decuce that and you would expect the cooler for the higher TDP card to have a matching higher conductivity cooler. Temperature != heat. Temperature = heat per unit mass.
A 4670 Ultimate without much airflow next to it can easily go to high 80s. A 7850 would probably go higher.
The Sapphire 4670 Ultimate has a pretty basic small passive cooler. Looking at the Powercolour, the cooler is over twice as big in heatsink area so it's fair to assume that it'll be able to handle over twice the TDP, hence no reason to assume that the 7850 will inherently run hotter because it's got a higher TDP. Wait for the first reviews but I wouldn't expect it to be that high. That cooler looks about a match for an Accelero in which case maybe expect 75C.
As long as this affects longevity (during or after the warranty period), it's something I like to consider.
Any proof that 85C in particular affects longevity? It's generally ancillaries that fail, not the GPU itself which will run way north of 100C (my record is 118C for a Geforce FX with a loose IHS and some Via processors are rated at 150C). Things like VRMs have heatsinks on many modern cards and for good reason, these are the things more likely to fail rather than the GPU. There's therefore no reason to assume that a GPU temp in this range indicates premature failure.