I'm so glad this post just resurfaced because I have the exact same question as the OP.
Eibach or Tanabe? I know the benefits of coil overs but not interested that much $$ right now.
I am puzzled by the specs the OP had. I've never heard any of these options are softer than stock.... That seems counter productive.
2010 G37s coupe.

Part 2.. Who's the best vendor for suspension? Looks like everyone's pricing on springs is about the same. Or does it even matter.

 

Tein H Tech anyone? How's the ride vs drop? I have Eibach PK right now.. not feeling the rear too much. Can still see a gap. Any suggestions are welcome.

 

Here is some updated info on IPL

Eibach PRO-KIT—2011-13 INFINITI G37 IPL DATA:

FRONT:
OE rate: 96.1 N/mm (549.0 lbs/in)
PK rate: 46.0, 96.0 N/mm (262.7 - 584.3 lbs/in )
Tubing: Yes
Bump-stop: OE
OE wheel center to fender: 374.5 mm (14.7 in)
PK wheel center to fender: 364.5 mm (14.4 in)
PRO-KIT will require additional alignment kit 5.72130K to align to factory specifications.

Front Alignment:front
OE Camber = -0.42 +/-0.75 deg
OE Caster = 4.83 +/-0.75 deg
OE Total Toe = 0.06 +/-0.12 deg
PK Camber = -1.0 deg
PK Camber = -1.2 deg
PK Caster = 4.9 deg
PK Total Toe = 0.06 deg


REAR:
OE rate: 79 N/mm (450 lbs/in)
PK rate: 65.0, 93.0 N/mm (371.2 - 531.1 lbs/in)
Tubing: No
Bump-stop: OE
Damper: OE
OE wheel center to fender: 387 mm (15.2 in)
PK wheel center to fender: 351 mm (13.8 in)
PRO-KIT can be aligned to factory specifications.​

Rear Alignment:rear
OE Camber = -1.25 +/-0.50 deg
OE Total Toe = 0.17 deg. total +/-0.17
PK Camber = -2.0 deg
PK Total Toe = 0.16 deg