looking at your bike, which I basically see as a alu-105 setup (albeit a good setup, Felts are lovely bikes), I can suggest a few things but right away I'll say I don't think there is a "magic bullet". In no particular order:
- wheels, as you say. On a lot of low-to-mid-range bikes, you just need to look at what wheelset they have to realise that this is where the manufacturer is keeping costs down. Obviously this goes away as you climb the range but whichever way you look at it, the better the wheels, the more expensive they are. For example I upgraded my wheels last year, but the ones I got cost me 50% of what I paid for the bike, over 1000GBP. (In fact I started off with near enough the same question as you just asked, and this was my answer.) So, relatively speaking, a lot. However, purely performance-wise, this is probably one of the best upgrades you could do. As with most things cycling, you're paying for lightness and aerodynamics.
- groupset. Your bike has 105 components, you could always upgrade your components to Ultegra. (I'm thinking specifically of the 10-speed mechanical Ultegra rather than the 11-speed or electronic versions here.) The plus side here is that you can do this in a piecemeal manner, so they will fit to your budget/timeframe, the downside is that these parts work out a lot more expensive if bought separately than they would if they came on a new bike. That's just down to economies of scale. A further possible downside is that you won't notice much difference! 105 ain't a bad groupset.
- keep saving until you can upgrade the frame itself. You always have the option to go carbon.
- how much are you interested in the actual mechanics of your bike? In terms of maintenance, for example. Some people hand their bikes off to the bike shop, other people do things themselves (or would like to). If you're in the latter category, a decent tool set perhaps? Plus of course this type of "upgrade" would last you for life.
- lastly not so much the bike itself but peripheral stuff like getting good clipless pedals, if you don't already have them, more comfy shoes etc. Perhaps, if it turns you on, a gps-enabled trip computer. Personally I love the "data" aspect, and a Garmin 800 was top of my "next cycling purchase" list for a very long while, of course it doesn't make me any better a cyclist but to be able to look at all my rides retrospectively certainly adds to my enjoyment. Perhaps also smaller bike parts such as a better saddle etc. Obviously there are no big bangs here, but if you're budget-driven....
I think which, if any, of these you choose is ultimately going to be down to how you see yourself using your bike going forward. Are you wanting to get faster? ride longer? go touring? compete? etc. etc. etc.
Likewise you could look at every suggestion and decide that none of them is worth it.
