I used to have a F/4-6.3 ! That's right, 6.3 at 300mm! Yet I still managed most things... it all boils down to the money of course. If you want to spend $1500, I'd recommend any F2.8 70/80-200/210mm lense. These types are what most of the serious photographers use. I have taken over 3000 photos with my trusty Nikkor F2.8 since last August, and I have been pleased ever since. If you're into aviation photography as "something to do", then any 35-200/300mm will do, preferably at F/4 or faster, but for every stop faster, the price triples.
Click for large version
Photo © Rob Rindt
This was shot with an 80-200mm F2.8, on Kodachrome 64 with an F90x, at F/8 at 1/500th-600th. This is why I leave the Kodachrome fuzz border so you can see the whole slide, un-touched.
It's hard for me to talk positively about my old 28-300mm lense, but for me the slow-speed wasn't too much of an issue, but more of an edge sharpness issue. If you take a perfect slide with an F2.8 lense, the slide will be crisp, and sharp edge to edge, and around the 4 corners, even if open to F/4, and even F2.8. On a cheap lense, the first thing you will notice is the 4 corners becoming blurry... this is when you project it and look at it very closely, and also, you may suffer from what I call "perplexing vignetting". It's when you see any edge become more "black" or darker than the rest of the slide... sometimes it happens in one corner, sometimes two, and sometimes all four!
Here are some examples of what I am talking about... Click for large version
Photo © Rob Rindt
This slide suffers from everything... edge blurryness AND vignetting. I would give anything to shoot it again with my F2.8. Also, as you may know, F/8 is the sweet-spot on all lenses. Vignetting and edge blurryness occurs most often when the lense is wide open, in your case, F/4.
Hope this helps you take some better pictures, and gives you some insight. Feel free to ask away... I hope I am qualified enough to help...