Improving the flow through the carbs with less restrictions will give you an immediate increase in upper end power, hot or cold air. It'll take a 5k rpm motor to 7k rpm if the head can flow well. Cold air will not make much, if any difference if the ouitside air temps are higher than about 60 degrees anyway, and the difference in power is negliable.
The phenolic spacers are very necessary for these carbs to draw A/F mixture well, making the flow path longer. IIRC, they are (were) available in different thicknesses to optimize the suction and carb piston rise rate. I have seen some in use on highly modified engines with spacers over an inch thick. A good start is some around 3/8 in thick. At higher manifold and engine compartment temps during summer months, they help to make the carbs run smoother because they keep the carb bodies cooler. I would recommend you get a pair and just use some red hi temp rtv and forget the gaskets, just let it set till cured with the nuts finger tight, like using 'form-a-gasket, but don't use so much that you get a bead inside the carb ports. I would scrap the heat shields that come with the HIF6 and make one that straddles the two carbs blocking the heat rising from the exhaust downpipe. Even if you have the HIF6, not the HS6, the car will run better the cooler you keep the carbs and fuel in the float chambers.
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'89 245 Sportwagon, '04 V70 2.5T Sportwagon