Is it possible to pre calculate execution speed of micro controller for a given setup theoretically?
Yes, it is possible, but you need more information than you provided.

You mention 9x sensors, i2c rate is 400 kHz; for a 16b register you need approximately 30 clock pulses, that means 75 µs; assuming you mean 9x axis and not 9x, 3-axis sensors, and assuming each axis is a 16 bit value, this means you will need approximately 675 µs to read all the sensors.

Then you want to send your stuff through serial, again the payload is assumed to be 16b x 9 = 144b, at 115200 baud (=bits per second) this will take 1.56 ms, assuming 8N1.

The total is a little more of 2 ms, that would be 500 Hz of loop frequency.

This assumes that the micro can move the data around as soon as data is ready, but with an 8 MHz clock I am not sure you're gonna be able to do that.

I would say that, given my assumptions, a safe bet is a 100 Hz loop; you probably can go faster, but 100 Hz should be easy to achieve.

If the sensor data is more (or less) than 16b then of course the calculation needs to be redone.