This may or may not be legal depending on what you want to do. On OEM seats this is not a good idea. With buckets, this is the ONLY proper way to do it.

 

How much do you reckon an LSD would cost including install and tuning?

 

Depends on the LSD, and OS Giken is about $1600, $150 in misc parts and $175 for a driveline shop to install it if you hand them the third member. Others diffs are cheaper, but the all require the misc parts and proper shimming.

 

And this would be a better investment than coilovers for sure?

 

YES*





*In my opinion. Again, it comes down to driver. Go to the track and see what bothers you first. For me the one wheel wonder was downright embarrassing midcorner when I know I could have rolled onto the throttle sooner.

Read up on diffs. I can't disagree with terrasmak's recommendation that the OS diff is the best clutchpack LSD for this car. Period.
However, depending on driver style the helical diffs like Quaife may be better. They act telepathically and let you get on the gas sooner than a clutchpack, although with less total grip than the clutchpack diffs can provide. There is no right answer. It's personal preference. Some platforms consider the clutchpack to be the ultimate all-out LSD while other platforms prefer helical to squeeze out the last few tenths.