It does increase pressure to an extent. Its more like the coolant is expanding as it gets hotter.
Downgrading to a cooler t-stat more than likely will not prevent your LIM bolt from leaking. The coolant is still going to get hot, and it will expand, which means it still has to go somewhere, and that one bolt thats not sealed is going to let it get by.
Some people like to run cooler t-stats in the summer time to keep engine temps down, and change them back in the fall-winter time.
If you think your heat isn't getting hot like it use to, it may be time for a flush of the heater core and or possibly the entire cooling system. Any idea from the previous owner about when it was done last?
1997 Pontiac Bonneville SEi-SCRAPYARD
2000 Chevrolet Blazer LT 4X4 4.3L
2002 Chevrolet *-10 LS Ext Cab 4X4 5.7 V8