Originally Posted by Marc Sabatella 
Longer but not faster is easy - as you say, the 55-300 handles that, the K300/4 even better. ............Indeed, it's pretty unlikely in many venues that they'd let you in the door with 300/2.8 without a press pass, and if you've got a press pass, you can be shooting from close enough that you don't need a 300 is the first place.
Frankly, for outdoor concerts or other settings where you need 200mm or more, I think you've got a great lens for that already. Sure, a 300/4 sounds nice, but 99% of the time, that's too long, so you'd want the 70-210 too. And it doesn't take *that* much cropping to get 210mm to look like 300mm.
...... if you're serious enough to be considering spending a few hundred dollars on a lens for concerts, you're serious enough to make the effort to shoot from as close as possible. It's pretty rare that I need anything longer than 135mm; I'd consider the DA*50-135/2.8 more or less perfect if I wasn't more attracted to small primes myself. But most of my shooting is in clubs, not auditoriums or outdoors. If I was mostly shooting in locations where I truly needed something longer than 135, I'd be looking at the 70-200/2.8's. But either of those zooms would push your budget.
If I were you, first thing I'd be getting is an old manual 135/2.8; that would supplement what you have (the 70-210 and 55) nicely and cover a lot of ground right there, and leave plenty of money in the budget to apply toward something else later.
Marc - thank you so much for your insight. I was actually hoping you'd provide them seeing as you've said that you're a concert photographer.
Just to clarify - these are shots that I envision taking inside my children's auditiorium or stadium while they are playing with either their marching band, symphonic/orchestra, or jazz band. I do also have an off brand (gold star, IIRC) A 135 2.8. I have not tried using this yet since I've been focusing on their outdoor venues and the 135 is not long enough.
I did see a 200mm 2.5 for ~ 400 and somewhat regret not pulling the trigger. On ocassion I've seen either a Takumar M42 300/4 or the K 300/4 with the Takumar running ~ $200 or so. What is the relative value of the 200/2.5 to a 300/4 or the 55-300? I'm thinking the 300/4 might be a good lens for marching band as they do often good stadium lighting.
Keep in mind that I don't have press credentials, although I could see trying to get one for the stadium venue as a volunteer taking pictures for the school band - this is why I'm a little more keen on getting 200mm or possibly a 300mm. I've even thought about trying to get an old Nikon 300mm 2.8 Ai to see if it'll mount on the K200.
Originally Posted by ballgofar Tamron Adaptall 300mm f/2.8. I just got mine a week or so ago and so far I love it. It's very big, so check out its specifications and make sure that's something you can deal with, but its size is nothing out of the ordinary considering it's a 300mm f/2.8.
I haven't looked into a Tammy or Sigma 300 2.8 but my instinct tells me that this is more than a 55-300. Seems that the 70-200 2.8's might be less expensive than the 300 2.8's