Don't worry, you will: The change from glow to spark, if done properly, will save about 20% (volumetric) fuel consumption when comparing on the same fuel composition as you were using on glow.
The change from Nitro to zero Nitro will save about half the percentage of the Nitro you omit.
Then changing from glow to spark, and leaning out to "2 clicks rich off peak" (that part does not change) you will notice another conspicuous thing: the slime will more or less dissapear, instead, you will see oil and water, separated.

The slime previous, was oil mixed with the excess unburnt fuel (that excess fuel is a necessity on glow ignition, in order to control ignition timing). This excess methanol allows ALSO the water formed during combustion to mix with the oil, which is the reason for the "slimey" properties of the residue.
This water and fuel content reduces lubricity of the oil, reason why glowfuels contain so much of that stuff.

On spark, and properly adjusted,this excess fuel is not present, therefore, the oil and water do not mix anymore, and the oil retains its lubricity. You can safely reduce oil content to 12~15% or possibly even less, and this volume can also be subrtacted from the volumetric fuel consumption.

So yeah, depending on your previously used fuel composition, you could possibly see a reduction in fuel consumption of about 25~30%, at the cost of a bit of power if you were running high Nitro numbers, and with a much ieasier to clean plane to boot.

Since fuel linearity of glow carbs is designed for glow, there is a slight possibility of a lean midrange, but that is not a given, and with 3-needle carbs usually not a problem.

Word of warning: it is noticably different to detect rich or lean conditions by ear, since spark ignition has a fixed timing, and the audible signs of lean and rich as you are used to with glow, are caused by retarding and advancing ignition due to mixture strength. Takes a bit of getting used to, but no biggie.