Handling is kinda bad after getting new rear tires - change the fronts or the brand
 

I changed my PS2 rear 18s for Sumitomo HTRZ IIIs but left the PS2s on the front (have about 50% wear).

Well, my once solid as a rock, super-responsive Boxster is an oversteer mess (or maybe it's understeer - it's just kinda all over the place).

Do you think I should replace the fronts with the HTRZ IIIs or just replace the whole set with Michelins, like the PSS tires?

I can't figure out if the bad steering is from the HTR ZIIIs or the the fact that I have two different brands, with one more worn out than the other. The steering was fine before I replaced the rears.

Sounds to me like you have mismatched tires. If new rear tires are the only change you've made to the car, then that is the only possibility. I'm not familiar with either tire model you are running, but it's likely that the new rears don't have as much grip as the used fronts (or vice versa). I bet if you get new fronts to match the new rears, your problem will be solved.

Did the same thing with Sumis rear and pilot sports on the front. My car developed snap oversteer, scared the crap out of me. Ditched the Sumis as soon as the tread was mostly gone which did not take long. Never had the problem again.

I replaced the rears on mine. It is handling better after a few hundred miles of run in. Also, when I checked the tire pressure one was about 32psi and the other was about 46psi.

I am running the sumi HTRZIII for the last month and LOVE them

You probably have very mismatched tires and the rears have more traction than the fronts

Go with 2 more Sumi's, together they are probably less that 1 PS2 and you will probably be very happy.

Also , you may need an alignment with new tires. Tire pressures may also be different I am running 32 f and 34 r with 225 f and 255 rear on 17's and it is the most neutral I have ever had it

I'm running the Sumis on the rear (265/35/18) and Contis on the front. I'll be adding Sumis on the front for next season. At the track I didn't notice anything particularly wonky about this pairing.
I guess the question is are you getting understeer (ploughs on regardless) or oversteer ( rear end tends to break loose). Or is it just wobbly. If this can be determined and clarified, it would be a great help for everyone who is making tire decisions. :)

Okay, I had the exact same issue under the exact same conditions recently.
Installed new rears (Primewell, cheapo brand) but kept my 50% tread Michelins on the front. Immediately super twitchy with snap oversteer. Downright scary to drive on the highway. After a few hundred miles of wear-in the instability softened quite a bit. Then I put new matching Primewell tires on the front and the problem went away completely. Back to a rock solid beast.
From my experience these cars seem to be hyper-sensitive to two things: Thin tires and mismatched tires.

Don't think it is the Sumis either. I use them and no issues when all are matched the same. I did replace only the rears last spring and noticed some squirmy handling in the back until I broke them in for a few hundred miles.
I think you have a mismatch in tire brands and wear.
My fronts are now in need of replacement and the car has become more prone to bump steer and needs a bit more attention on the Hwy in a straight line

Get the fronts in line with rears

i agree with getting some miles on them. in my experience new rears will feel a bit greasy initially, as well as introduce a bit of rear bump steer (which will feel like an issue with the steering); it should all settle in after a bit.

Just finished 2 track days with Sumi HTR ZIIIs and I was very pleased with them. The car was pretty neutral and I didn't see much if any understeer until after several sessions and they were really hot, much hotter than you will probably ever see with street driving.

If you are seeing a lot of understeer, try adding a few PSI to the front tires. Porsche builds in understeer with the factory pressures.

Also look at the alignment

Sounds like a pretty big tire mismatch and I would expect the car to snap oversteer easily. Mixing tires is fine as long as they are very similar in grip and responsiveness. Ps2 and Sumi are quite different which is why your car is so twitchy. Go with the same tires on all 4 and it doesn't matter which brand you choose.