Aldolfo,
We were competing in an air race where engines had to be stock but airframes could be modified with STCs. My Mom would put on a mod to see if the aircraft top speed was improved. If she determined that the mod had any speed improvement, it stayed - don't know if she got the advertised speed improvement. She was pretty sharp on this and holds the record for the most pilot-skilled cross country speed air race victories. The reason I share this is a lot of people go for these mods to fly faster and the plane did fly faster, I just think that if you are going to spend money, it is better to add fuel tanks because anytime you can pass on a fuel stop, that is a huge time saver and therefore the best speed enhancement.
We have aileron and flap gap seals, wing root fairings.
The rudder seal went on and slowed our PA39 twin down so it came off.
Twins are notoriously hard to get a smooth landing out of. Either it was these mods or the tip tanks but husband and I both noticed the Twin was easier to land smoothly after these additions. I'd like to think we got better as pilots

but it is more likely to be the mods.
Hope this helps.