(Mike Sertas answered most of this, but let's pull it together.)
On Earth, plants are found about as high as they could be -- "the highest-altitude plant species is a moss that grows at 6,480 m (21,260 ft)" according to Wikipedia. This is likely more limited by temperature and just how few opportunities there are to grow higher -- but let's use it as a starting point.
The scale height of a Martian atmosphere with the same temperature, composition, and pressure as Earth's goes with the relative surface gravities, so it's about 2.64x higher than Earth. However, our reference pressure is a bit lower, so we'll scale by that 0.975 factor to have equivalent pressure at 2.57x the height of reference. Therefore, our plant that is comfortable at 21,260 ft (6,480 m) is comfortable on Mars at 54,630 ft (16,650 m). This is from whatever altitude the 0.975 pressure holds -- if. we want to call that sea-level pressure altitude 11.7 km as the question suggests, this puts our moss at around 28.5 km above the lowest point on mars.
Let's sanity check this, and for this we have to get into "sea level" references a bit. Martian reference altitude is defined today somewhat arbitrary as the 610.5 Pascal isobar. The lowest point on Mars, Hellas Planitia, is about 6 km below this; so your sea level is 5.7 km above the Martian reference altitude. So our plants are growing at 22.5 km Martian reference altitude.
This is an interesting number, because it just exceeds the altitude of Olympus Mons at 22.1 km.
So, recap: We used a somewhat conservative starting point for where plants can live, because it's likely that plants on Everest are as much limited by temperature as by pressure. Looking at a Mars as your described, we modeled the atmospheric density variation with altitude and found that our Everestian plants would be happy (barely!) at the top of Olympus Mons. In summary, due to the scale height of the Martian atmosphere, if you manage to build an atmosphere that is approximately Earth-density at some reasonable sea level, there is no altitude limit to plants at all on the planet -- not just the base of Olympus Mons is possible, but even the crown.