Would I get better results (lower rpm's at idle and better fan speed "response" on demand/full load) if I'd invest quite a bit of money and replace all the fans with newer pwm fans from noctua?
Yes you would get better results from using PWM fans. The Asus BIOS Silent profile for example holds PWM fans down to a minimal duty cycle (starting at 20%) until the CPU temperature hits around 40C. With the combination of the Silent profile and Noctua NF-F12 as a CPU cooler fan you would see idle speeds of around 350-400 rpm. The Noctua NF-P12 makes an excellent chassis fan. Ignore the chassis fan headers and run NF-P12 PWM fans as a PWM chain from the CPU fan headers. Your motherboard has two CPU fan headers that makes this easier. Noctua supply PWM Y cables and PWM extension cables with their fans so this can be done without the need to buy extra items which helps offset the cost of Noctua fans. Using NF-F12/NF-P12 PWM fans in combination with the Asus BIOS Silent profile would give you a system that would be extremely quiet at idle or under light to moderate system loads.
To give specific details the 3 pin Noctua NF-P12 delivers around 800 rpm at 7V and this is the lowest speed possible using the Asus chassis fan headers. Using the Asus Silent BIOS profile and the Noctua NF-F12 as a CPU cooler fan I can calibrate this fan using Fan Xpert which on my system delivers these figures (duty cycle/fan rpm):
My current idle speed is 350 rpm with a CPU temperature of 31C and system temp of 34C. According to Fan Xpert the NF-F12 would not hit 800 rpm until around 55% duty cycle when the CPU temperature would have to be 52C. This is with a single NF-F12 as CPU cooling fan, intake and exhaust NF-P12 PWM fans all run as a chain from the CPU fan header and the Asus BIOS Silent profile. I run Fan Xpert under Windows only as a monitoring tool not for fan control.
Similar results could be obtained with any of the fans that can get down to 300 rpm under PWM control. This would include some Scythe SlipStream/GlideStream fans and the Be Quiet! 120mm PWM.