I have the 50-135 and do a lot of portraits, both available light and studio.
I also have a 55mm (not a DA but an F1.8 M model)
Yes the 55 is a nice length but it is limited in its application, but gives you that shallow DOF (also it may give you the effective FOV of an 83mm, but it is still a 55mm lens and will act as such)
Personally, I would go for the 50-135 as it is a lot more versatile and it is'nt a compromise. In fact I would go to say it is the finest Portrait zoom out there.

You finally chose the 150-500, Good choice there.

Look forward to seeing the Safari results.

I only ask, as some lenses I have have been perfectly fine with the DS/K10D/K20D, but with the K-5 images are softer or the AF cannot cope etc. Ironically my Prime lenses are still stunning!

Specifically (so far)
Sigma 55-200 DC F4-5.6 (Budget lens, so not surprising)
Sigma 170-500 DG APO - Can't keep up with the AF in certain circumstances
DA 16-45 F4 - Sharpness trail-off to the edges, even at F8

The sigma 24-70mm EX F2.8 and DA* 50-135mm are more than a match for the K-5 though.

It's very close, but it depends on the camera and focal length.
The body IS of the K-5 is a little better than the K20D.
As for lens OS and body IS, the big difference is that with lens OS, you can see the stabilisation working in the viewfinder. But when it comes to results, there is not much in it. The Sigma 150-500 I tested at focus I took the same shot with OS on one and IS on the other. Results are just about the same. nothing in it really. But, as I say, the advantage of lens OS is that you can see the correction through the viewfinder, however up to 135mm there is nothing in it, longer lengths is where it is better.

Now to that mk2 Sigma 50-150. Its too big, thank god I have the 50-135.....

Whilst I love my 16-45mm I think I have gone beyond its limits with the K-5, It just can't cope with the resolution demands of the K-5. It's still great with my K10D/K20D, but the sharpness tail-off to the edges is noticable on the K-5, even at F8. However, it is not surprising really, as it was the first DA zoom that Pentax made (apart from the kit lens).
However, I still enjoy using it as it is so light and on the whole sharp, but on the K-5, it is showing its age.

wiki's are good, we use one at work for sharing info.

As For Wikipedia. It is FAR better than it was, and on the whole the information is accurate. As it has matured as a lot more controls are in place, including citations required for information. It is still vandalised occasionally, but pretty good now - what you need from an information portal.

You did'nt say what you have at the wide end.

But lets assume you have something like the 16-45 or eqv.

The long end is where it matters most for safari.

I have the 170-500 and most if the time it is great.
But I tried the 150-500 at Focus, and I think I will be changing.

So, Keep the 50-135, you will kick yourself for getting rid of it. Sell the 55-300 and converter and buy the 150-500.
Then go to Africa with three lenses, a wide zoom (16-45?) short fast tele zoom (50-135) and the long tele zoom (150-500). You won't need much more than that.

Hope this helps.

I know I keep harping on about it, but the DA35 is a stunning lens. It is Pentax's little secret. Sure it has a plastic mount, but the lens is so light it really does'nt matter (unless you are very heavy handed with your camera) So, it's only F2.4, but it is so sharp and wonderful bokeh, it gives the 31mm Limited a real run for its money. Yes, the 31mm it probably the finest Pentax prime out there, but is it worth 7 times the DA?

It's a bargain and then some. Well worth £150 of anyones money.

Depends on the lens, my 35mm and 50mm have collapsable rubber, the rest are rigid. Thats how I like it.

May I ask why you want to shoot at 1/180sec anyway? I have done a fair few studio shots in my time, initially we had the light at some considerable power and shooting at F11-F14 at the aformentioned 1/180sec. most of the time it was too much, but we were inexperienced at the time.

Last weekend we were in the studio and we set the light power at F8 1/125sec. So much more controllable. Even a friend of mine had a Pentax 6x7 which only syncs at 1/30sec so we had to adjust for him too.

Now to the OP and his portaflash units. Yes wireless is the best way (I have a couple myself), but you will need a connector that is mini-jack rather than standard jack of Bowens, Broncolour, lastolite et al, as it won't fit.

I have used Pocket Wizards, Calumet and Yongnuo wireless. And apart from the Calumet triggers, the others are just fine.
If you get a black area on the edge on the images at 1/125sec, then there is a fault with the flash unit and not the camera or wireless. Why? We had the same problem with Pentax and Canon cameras on Saturday, and the flash unit (a Bowens) was at fault.