Realistically, yes you do need HTTPS Inspection for proper IPS functioning. Without it IPS will still be able to inspect cleartext payload data, packet headers, and the initial client/server negotiations prior to the start of encryption in the case of HTTPS. Given that around 85-90% of web traffic is now HTTPS you will be missing a lot. This is why HTTPS Inspection was extensively covered in my IPS/AV/ABOT Immersion class even though it is not directly related to those features. Also don't forget about SSH, Check Point firewalls do support SSH Deep Inspection and the cool ability to block port forwarding, but this feature can only be configured from the CLI of the gateway which is why few people know about it.
Performance is definitely a concern, but at least some of that can be mitigated with a properly ordered & configured HTTPS Inspection Policy to avoid multiple HTTPS Policy lookups for the same connection, and avoid unnecessarily triggering Active Streaming when it is not absolutely needed. See here: HTTPS Inspection Policy Rule Order
At some point the Lightspeed/Nvidia card will probably be able to handle at least the HTTPS bulk encryption & decryption aspect, as part of this initiative SecureXL is making the transition out of the kernel (sim) to User Space (UPPAK/usim) starting in R81.20 on Lightspeed systems.
The unofficial way to estimate the performance impact of HTTPS Inspection is to cut the rated throughput of the appliance in half. So if an appliance is rated for 3Gbps with the features you will be using(NGTP,NGTX, etc), to account for HTTPS Inspection cut that number in half to 1.5Gbps.
Attend my online "Be your Own TAC: Part Deux" CheckMates event
March 27th with sessions for both the EMEA and Americas time zones