It does sound like you have a lot of water coming in. I know you are asking about pump advice, but I'm going to throw out some other suggestions. Have you considered trying to improve the situation from the outside? Things like extending downspouts farther from the house, and extending the sump discharge line. Landscaping improvements. Overhangs to reduce rainwater collection in welled exits, etc.
Next time you at one of those peak conditions, test you current backup system. Pull the plug on your primary pump and see how the battery power solution performs. Use the results of that experiment to determine how it holds up and if you really need a higher capacity backup. I think this would be the true test to see how you stand with a battery backup setup. You might need to add more batteries for your desired run time.
No personal experience with them, but it seems that Simer pumps don't have the best track record. From what I can tell, they were due to switch failures. As long as your upgrade is a reliable switch, you may have made major improvement by not using the Simer switch. Even so, you might want to have a spare for your primary pump on hand. Something like the Zoeller N98 might be better option. It is 1/2HP, and nearly same capacity as your 3/4HP Simer.
Another thing you can do is to get out your pencil, paper, and calculator to determine how much water is discharged with each cycle of the pump based upon the calculated volume of water using the diameter of the sump and the difference between the pump on and pump off height of the water. You'll have to look up the formulas. Or you could bucket fill the sump to see how much is pumped with each cycle. Add enough water to get it to cycle once, and then start filling and keeping track of how much water you need to add to get it to cycle. This with the shortest cycle interval will give you your actual flow rate. To use the rated flow rate of the pump to estimate the actual flow rate you would need to know how much time the pump is on during a specific time period. The on time divided by the off time will determine the percentage of the rated output the pump is pumping. That assumes the factory numbers are accurate.
Thanks a lot for the additional advice, it was however nearly the exact steps I took before arriving at my status today. The outside is graded well, if not over done. I have only 1 downspout that drains more than 10ft from the house, the others along with the sump discharge actually discharge at the end of my lot via underground PVC pipe.
I do have a back up main pump on stand by but it is another Simmer, a 1/2 HP version. Their switches (also under the Flotec name) are absolutly GARBAGE! You're right!!!
The battery backup has not been tested at peak because I tested it during a few mild flow times and all it did was hold the level steady, running non-stop during the 20-30 minute tests. In all honestly, I didnt test exactly how long it would run, I didnt think I needed to knowing it was working hard just holding a steady level during the "mild" tests.
In short, I think I am very well set up for every possible emergency situation except when I would be out of town and I've taken every step possible trying to find possible causes for the extreme flow at "peak" times. And its this peak time situation that I want to over come. With the combination of the battery back up and a water powered pump installed (probably in Fall) I will exceed any peak flow I can estimate.
Edit: I read some concerns about water use and cost, and I used an extreme example of the water powered pump running non-stop for 10 hours @ 15 GPM. Milwaukee water cost on average is $3 per 1000 gallons, which is very near the national average.
10 hours = 600 minutes.
600 minutes @ 15 gallons per minute = 9000 gallons
9000 gallons at $3 per 1000 = $27
$27 (max) for peace of mind during a very rare emergency? Sign me up.