Ichigo's shikai form, unleashes imense riatsu, and bankai compresses all that riatsu, so his bankai would probably be better if used sooner.
Increasing riatsu does increses ones abilities, but ichigo's bankai makes use of the compressed form, by increasing speed, strength, durability and lastly defenses. This is shown by the different blades in both shikai and bankai, The body long wide blade Zangetsu shows immensly spread riatsu that even burns the air around it, which is taken in by the blade and then upon a swing it amplifyes all the riatsu and shape it in a strike known as getsuga tenshou(Moons fang).Bankai when activated releases all raitsu suddenly then absorbs it, which then shapes his own appearance, the compressed riatsu gives ichigo a solid base increasing his speed & strength to the top most lvls, In his bankai form ichigo's shunpo or plain movement is comparable to Yoroichi's shunpo, and getsuga tenshou as black getsuga at a much powerful state,
Ichigo's and Kensei's zanpakutohs are perhaps the best suited for a hollow tranformation,
Ichigo's Vizard form takes Ichigo to a much more greater lvl., Normal shinigami uses there own riatsu to use techniques and stuff where as hollow absorb the surrounding the riatsu in the atmostphere, thus 2 power sources for ichigo, now u can guess how will it effect ichigo in bankai all the riatsu he releases absorbed back and compressed for usage. and being vizard is like being in cammand of some1 elses body, now this massively increases ones durability and life force. Kensei's wind type zanpakutou and his killing strength in vizard form together with telekensis makes kensei virtually the strongest.