well your expectations are a bit off.
The car doesn't have a roof. It will never drive like an STI
But given that I also see you havent' driven allot of other convertibles because this Boxster is as tight as any drop top I have ever driven.
Take a ride in the m3 convertible and tell me if you still miss the EVO.
Allot of mixed opinions on the short shift kit. Most 911 owners love them but not such unanimous feelings in the Boxster gearbox. Maybe its the differing engines.
The gear box is definitely a weak point in this car. Sound like your prior experience make a short shift kit a good experiment.
The strut brace is deffinitely a good idea and shouldn't be king's ransom.
p.s.
Is your car on lowering springs? Might want to look into Pss9 and raise the ride height. if the springs aren't stiffer than stock.
also maybe a lighter set of tires will help. That's more important than light wheels.