Reagrding the home tuner and for what it's worth.
Your best bet is the Maccari E-3650 spring, the small Apex seal #APV65798 or Tesla along with the Maccari lubes. The E-3650 spring is a drop in spring that has an inside diameter about .010 less than the stock Gamo spring and though not a perfect fit spring is very good if you are not equipped to make guides. There really isn't anything wrong with using the factory tophat and it fits the E-3650 spring ID fairly well also. Using the Tarantula spring is another whole different story.
The one piece machined tophat solid tophat has both advantages as well as disadvantages. The advantages are if machined properly, will fit the piston better as well as the spring helping to center the spring and reduce canting. If the proper weight figured into it, it can help in increasing the velocity some but in many cases not worth the it because of the increased harshness. A lot depends on what spring and power plant it will be used for.
The disadvantage is that usually, depending on the weight, spring and seal, it will increase harshness in the firing cycle (and one of the reasons why Gamo went to the light weight tophat), some not so bad but others quite noticeable. Also, it does have a tenancy to increase detonation, especially if there is excess fuel for combustion. A good weight is about 45 grams.
Unless done properly and fitted to a given power plant, using a heavy solid tophat is not the best way to go.
