ok with my actual HD 5850 I reah 395MH/s with 1000/300 at 1200mV (80°C)
I wanted to push a little and reach 400MH/s
You can probably reach 400MH/s by playing with the software rather than pushing the card further. I've found setting which will give me 413 MH/s for a 1000MHz 5850 but you might have to pull that back a bit if your rig is not a dedicated miner. Let me know if you want details.
Do you have the latest miner kernel code? There have been some improvements in the last few weeks.
I use GUIMiner (last version) with those option :
-v -w 256 VECTORS BFI_INT AGGRESSION=13 FASTLOOP=false
Is there something to change?
The settings you are using are very good for these cards using phatk (I can only assume GUIMiner uses phatk). Now that I see that these settings are good I ask whether or not you are using SDK 2.1. For me SDK 2.1 is a fair bit faster than SDK 2.4 (with either Catalyst 11.5 or Catalyst 11.6). If you are currently using SDK 2.4 then I suggest you give 2.1 a go.
Another thing to consider is whether or not the read-out from GUIMiner is taking a moving average before displaying the hashing speed. I know phoenix does this by default and I have to pass it '-a 1' to make it stop. If you manage to remove the moving average filter from the readout then you may see your hash rate jumping up and down, perhaps by a few MH/s. Try to reduce this jitter as much as possible by closing other running programs and/or reducing desktop effects/resolution if your card is driving the GUI.
Finally, because your clock is as high as 1000MHz you may find you gain another megahash by increasing the RAM frequency to 350 or so.