Thanks very much for your response - I thought for a while you were managing to keep your 400Mhz miners under 60°C in a warm ambient environment! 
It was the hottest day so far, when I took this screenshot. Because of the mods you'll get smaller temperature differences between the chips on the intake side and exhaust side, and you also get better cooling performance as you get more air through the heatsink, thus extending the lifetime of the miner and especially the chips at the back side. If you want to play extra safe because of the warranty, just install the mods and keep the miner at 350MHz. It will be optimal for your miner.
I'm pushing mine very hard. So far without dead chips or other issues. I've found that some miners can go to higher frequency than others, before their hash rate starts falling.
Currently my 400MHz miner is at 58/62°C because it's ~25°C in the room.