Not much will grow as fast as euphyllia. They are almost like weeds of the LPS world, so, you may be comparing growth to the wrong corals. SPS should grow fast, but 2 AI Sols is nothing for SPS, especially at just 50% power, unless they are directly in the hotspot. The AIs, for shallow water SPS are only really good if spaced 18" apart and still on more than 50% output. On the other hand, if you had 3 SOLs, which is really ideal for a 90g, then you may have too much light for some LPS and they may not grow as well, unless you are careful about placement (i.e. putting a chalice in the hotspot of a SOL at shallow depth may melt it LOL). That's one reason why mixing corals is tough, they come from very different environments and are accustomed to different light intensities and spectrum. Some of this can be overcome by providing extra light and acclimating the corals to it; usually corals can adapt to more light than they can use, but if they get less light than they can use, they won't grow well.
I agree, you may want to borrow the clubs PAR meter and test PAR levels. It can vary quite a bit, but generally speaking, most SPS shallow water SPS will probably do best with around 300 PAR. Deeper water SPS more like 100-150. LPS can vary quite a bit, acans, chalices and others that like lower light LPS, probably do well 50-150. Euphellia probably 100-300. Clams 600+. These are kind of generalized ranges, ideally you would match the PAR for a given coral to the corals photosaturation point. This can sometimes be found in the published data. Alternatively sometimes hobbyists can tell you what they have had luck with although more anecdotal. Again though, usually erring slightly on the side of too much, will give better growth as long as your careful about acclimation. Also, going substantially over the photo-saturation point becomes stressful. So, balance is always good.
