On the one hand, I'm not sure I would trust a charger like that with brand new NiMH packs. On the other hand, I have seen plenty of brand new GP3300 packs take a charge of even as high as 4000mAh the first few times. I would watch the temperatures; as long as you are not overheating the cells, you should be OK.
However, since you invested in new packs, I would really recommend getting an adjustable digital peak charger that is more suited to NiMH cells. The Duratrax ICE, for example, sells for around $130 and will do everything you'll ever need. There are many other options from Reedy, LRP, APS, Competition Electronics, and so on, that all sell for about the same.
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