you must have a luxury version of M5, I didn't own an M5 before, maybe it's because of the electric pump?
There is no luxury version of the M5. An M5 could have been optioned with certain convenience features but there is no version of an e39 M5 that is specified as "luxury."
When I say light, i'm referring to the amount of effort required to turn the wheel and the response you get at speed. Obviously light effort at parking speeds is fine, but at higher speeds I would prefer a much lower input to response.
This is exactly what the torque sensing variable assist rack and pinion steering system does: light at low speeds and more effort at highway speeds. The MDX's steering ratio is 17:1 to the X5's 19.5:1. The MDX, inherently, is lighter. However, when loaded under cornering it's still fairly communicative.
It also does not come across as linear through the speeds....take a high-speed cloverleaf exit and as you're dropping speed around the turn the steering gets lighter at a much faster speed than you expect. It's not as obvious if you're bleeding speed fast before a turn...but if you're dropping speed and turning at the same time it doesn't impress me at all.
Again, this is an example of the variable assist steering. As you drop speed and RPM, the steering becomes lighter. If you're trailbraking into a clover leaf/corner, that's an ideal time to rely on the SH-AWD system to pull you through the corner. Acura engineers chose the X5 as the standard to measure against, and IMHO (as we obviously have different experiences and opinions) the MDX is superior in linearity (across the RPM range), equal in feedback, but trails in precision.
Clearly these are designed for people that aren't hot-lapping their SUV's...which is probably a good thing. Very tight and reponsive steering on an SUV can cause problems in a hurry for the average un-trained driver. I have no doubts customers have complained...the Odyssey took a performance hit on the latest re-design as well...as have many Hondas actually. Nissan is even starting down this path it appears. Everything is about gas mileage and volume sales!
Actually, the chassis, suspension, and entire driving dynamics were honed on the Nurburgring - the toughest track in the world. At it's very core, the MDX is a performance SUV. Because of it's variable assist function, the MDX's steering is responsive and linear at every speed range. In my experience, a company doesn't spend the time and effort to optimize their product in a particularly tough way because of penny pinching. In fact, that Acura went through great lengths to meet or exceed the driving dynamics of it's euro competitors is evidence of this.