The spitfire serves as a supportive and complementary class to the other aliens, bringing new capabilities to the table, while still not overshadowing the importance of the other classes. It is my hope that with the introduction of the spitfire (and the corresponding changes to the evolution tree), we should see more variation in alien class usage, more variation in human response, new strategies emerge, the dynamic should be very different, and the aliens should be less overpowered against humans compared to before.
The basics of using the Spitfire should be straight forward, while advanced usage should require mastering new skills.
At the moment the Spitfire replaces the the stage 1 basic goon. It also counters jet campers and gets to hard to reach locations, but at the same time jets can be useful against it (as the jets can't be zapped if they are not grounded, but they can still be air pounced).
The zap doesn't seem to be over powered against buildables, the rets can only be zapped while the rets can shoot back at the spitfire, So generally speaking, you would not have to worry about the human base being annihilated by the spitfire because the humans stepped away from the base for moment, any more than you would have to worry about that from the basic mara, basilisks, and dretch. With skill, the zap does have a chance to deal some meaningful strategic damage against buildables, and can be used to support attacks. It will be interesting to see what changes to the human building strategies will have to be employed.
As I've mentioned, the assets for the Spitfire is at the moment from yalt's (aka Matth's) Tremulous GPP mod Edge. We're using those assets at the moment as a placeholder so that we can test and develop the game play aspects of the Spitfire now (as its game play and coding is my top priority for the Spitfire at the moment), and later we are going to have to have new assets made for it that would best fit with its new developed role and functionality. (Not to mention the current assets are "non-free" and we will want to have assets that are licensed cc-by, cc-by-sa, public domain, or equivalent).
I would like to add that while basics of the initial implementation is based in part on the code from yalt's mod for the hummel, the flight movementof the Spitfire is already very different, and there are a substantial amount of new/different features/enhancements. I was very well familiar with the strengths and weaknesses the hummel had in a game play perspective (from playing and mastering it for many years), and I did get a good amount of inspiration from it as a starting point for what to do and what to not do for a flying alien.
Please elaborate on and clarify your question.