Well for one thing, all you'd hear would be bass
By "highs", I have to assume you mean the mids AND tweeters (900w to the tweeters alone would SMOKE 'em!) The mids need a fair amount of power. To figure how much power you need (or coversely, how loud they would play with 40 watts), you need to find the sensitvity of the speakers (no, not whether they have feelings or not, but how efficient they are ). This is usually expressed in terms of decibels/watt like 90dBs @ 1W. From there, you double the power for every additional 3dBs, i.e.,
90 dBs @ 1w
93 dBs @ 2w
96 dbs @ 4w
99 dBs @ 8w
102 dbs @ 16w
105dBs @ 32w

etc, etc. Do this for the sub and the mid/highs to see how much power you need to balance the volume between the two.

But before I went to a smaller amp, I'd just pop in an Optima yellow top. You can find 'em online for $119 or so.

If your talking about power draw then it will depend. You will have to look at the specs to see what kind of current draw they have. A 40X4 amp would be plenty to power your system if tunned properly but if its a good amp it will most likely draw more current than the one you currently have. If you compare a cheap amp with a quality amp the power ratings cant be compared. A good quality 50X4 amp will out perform a 100X4 cheap amp all day long. I am talking about rms and not max wattage as well. A good amp will always be louder and cleaner due to quality components and a more accurate spec ratting. There are things like the low level output and supplied voltage that will effect the way the amp will perform as well. If you have the money than get yourself a cap and if your battery is old than a optima is a good choice and they last a long time. The red or yellow will work fine so go with what you can afford. Your system is fairly small so there is no need to go overbored.

Anyone kno which Optima Yellow top fits a 95 SC400 cuz the optima web site says it dont fit.

a lot of ppl use 2x4's and stack a pair under the optima to get the height just high enough so that you can use the stock battery tie. it works

I really meant to say what model # yellow top fits the SC400.