Re: Tharsis Planum Mars caused by Hellas Planitia impact ?

Actually, the idea that the Hellas impact might have caused the Tharsis has been tossed around a bit. Alas, it doesn't really seem to work out, as nice as the theory sounds at first.

There are several reasons to think that the two events are unrelated:

1. The two sites aren't really antipodal (on opposite sides of the planet). Hellas basin is located at about 65^° east and 50^° south. Tharsis is at about 240^° east and pretty much on the equator. So while the two locations are about 180^° of longitude apart, they're 50^° of latitude off. Is this a lot? Yes, it actually is. If you imagine drawing a circle with a radius of 50^° around the antipode of Hellas, you'll end up enclosing about 20% of the planet's surface. So I can easily believe it's coincidence that the two are as close to antipodal as they are.
2. Timing. While it's difficult to get an accurate date on Tharsis, it seems that the most recent eruptions are relatively recent events, perhaps occurring as recently as 20 million years ago. They are almost certainly from the past billion years. Hellas is much, much older than this, at around 3.9 billion years old. So if Hellas caused Tharsis, the effect would have had to continue to cause Tharsis to erupt over billions of years. (What could prove to be even worse for the theory, I've seen some dating schemes that indicate that Tharsis started before Hellas was formed. Unfortunately, dating Tharsis without being able to drill down into the older layers is tricky at best.)
3. There's no known mechanism to go from an impact to forming a large shield volcano across the planet. Tharsis is big. Really, really big. Even an impact as big as Hellas wouldn't be enough to pop Tharsis out of the other side of the planet. To form something that large, you need to create sustained volcanism there, which requires moving a lot of material in the interior of the planet. That's takes a lot of work, and it doesn't seem likely that an impact packs enough punch for it. So for, models agree with this intuition.

All of this isn't to say that you cannot get effects on at the antipode to an impact site. On Mercury we see "weird terrain" opposite the Caloris Basin impact site. On the Moon, remnants of a magnetic field are found clustered opposite impact sites. But it's important to note that these effects are superficial: they don't seem to affect the deep interior of the planet, just the thin surface layer.

Another point worth mentioning: I doubt if you can accurately extract the impact angle/direction from which the Hellas impact arrived. Most planetary impactors arrive at hypersonic speeds so that their craters are not carved out, but instead are more of a blast effect. One of the results of this is that impactors leave circular craters, regardless of the direction that they arrived from. (There are qualifications to this, but this is true for the majority of impacts.) When you see non-circular shapes, this is generally telling you more about the underlying geology of the site than about the impactor. (Barringer (aka Meteor) Crater in Arizona, for example, has a somewhat square shape due to fault lines in the area.)