Speed Plus Materia only does so much
I agree raising a stat to 150% of it's original for each one is a very marginal effect.

Strength/HP only decreases from Magic and Summon Materia, of which HP Plus makes the strength trade off the only one of significance. Support, command and independent materia all have no adverse effects. With intelligent materia set up you only need a few pieces of magic & summon materia.