Don't get me wrong - some of the mods are really cool (especially to look at).
The people who could tell you more would be the owners group around your aircraft - I guess they'll be in the USA.
Since you will gain probably only 5-8 kts total out of the mods it is best to go for the handling mods or soundproofing. You will still spend a similar amount of time in the aircraft so you might as well look into soundproofing issues in the form of new windows and materials within the upholstery. If it sounds too good to be true it probably is.
We completely revamped the cockpit - lots of money but it updated the aircraft to almost brand new condition.
Good luck with your project.