Is an upgrade from a 7870 to a r280x worth it?

you are really the only one who can determine if its is worth it...but if you are happy and can wait its almost always best to do so. I had a 6970 from day one all the way up to about a month ago when I got a 7950. To be honest I could have pry waited longer but I saw a good deal for one at 215 so I had to take it.
 
Not really worth an upgrade to 280 IMHO, 290 yes if your going to be OCing using multiple monitors and things of that nature you would definately need a cooling system. If your using a single monitor and small OCing you could again in my opinion get away with not having to have a cooling system. My reference is my 770 never had nor wanted to OC it. I play everything on Ultra settings I've never even heard it scream at me. Use 1 monitor about to have a second in 3 weeks. The 290 is .1% more noisy than my 770. Performance wise its a No on the 280, a definate yes on the 290, but like you the temp is an.... interesting trade off but if you are using it stock or slight OC and 1 monitor you should be absolutely fine temp-wise. I would skip the 290 and go GTX780 for $70 more or probably less than +$70 now. Better performance than the 290 and you wont be spending more on a cooling unit. Anyways the 290 is a great card if it fits your gaming wants taking in consideration what I've stated then go get that beast.
 
I saw Asus is about to release their 290 if its like their 280 itll be direct CU which will help drop the temp. Other than Gigabyte I believe Asus is the ebst at keeping GPU's cooler than the other's.