I'll second what Qemuel said, you can't get the 4 hits on Increase Reflexes necessary for the +3/+3 without casting at Force 4+.
Same goes for Improved Invisibility, you cast it at Force 1 to save on Drain costs and roll five 5s and 6s on your Spellcasting Test, you only get 1 hit. Then, security guard only needs 1 hit on his Resistance Test for the invisibility to be ineffective.
Additionally, something to never, ever, forget... Force is added to your dice pool when someone tries to dispel your sustained spell. Same goes for passing unwittingly through mana barriers. See pages 185 and 194 for the rules.
So, the short story is, it doesn't matter how little effect Force appears to have, it is something to never skimp on for your important spells.