10K for diagnosis is steep but Iāll suggest let them diagnose it. Find what they have to say, and then go for the gearbox replacement from the other workshop.
One more thing, what do you mean by RPM drop? I have two civic turbos. One is at 38K and the other one is 10K driven. Both when A/C is turned on drop rpms to 600, for a split second sometimes but only in Drive or Sport Drive mode. In Neutral, thereās never any rpm drop.
So maybe you are overthinking about the RPM drops? Or just really really feeling it?
Btw, the 40K driven turbo is still with the original sparks plugs, cat never cleaned, to this date anything after factory on it is the tires. So the purpose of telling you all this is, if you have replaced everything and it still is sluggish then thereās definitely wrong with your car right?
Have you done any compression tests yet on the engine? I highly doubt that itād be the engine but it wouldnāt hurt doing a compression test. The gearbox oil though, should have been full unless the previous owner skipped the 40K marker for the oil replacement.
This car doesnāt have a dipstick, the only way to see if oil level is full is by removing a level check bolt located right above the drain bolt. You remove this bolt, and the oil should start ādrippingā out within 3-5 minutes. Note that your car has to be warmed up for this. If it takes longer than 5 minutes before oil drips out, your oil is low and vice versa. There is absolutely NO other way to check the oil level.
Lastly, you have the chance right now to do a good diagnosis before you replace something or take a decision. So do it. Be sure what to replace and what not to.
I always do my diagnosis first, I DIY 80% of the jobs. And then I have my friends who have workshops, after I complete my diagnosis I go to them, and tell them what needs to be done. By Allahās grace I am right almost all the time 
. You should too. Youāll get this solved, thereās no doubt in it. Good luck.
