I was advised to look at Oberoi for rental returns - rate per night would be something developers will be predicting to clients.
I would agree however Sahl is now somewhat overpriced for the place with so much construction and no infrastructure yet. When it has - it will be more expensive than El Gouna. But always take into account Sahl Hasheesh is freehold properties and El Gouna - Signature Validity Verdict only.
They are about to open the Oldtown this year, so to my mind the price increase can be predicted after this. But in the end investing in Sahl is already for people who have substantial deposit to put in for investment - you will deposit more, you will get more in the end.
Or you can go for small investments in Hurghada and make use of them faster.
People say Gamsha Bay is great for investment and even not yet started - I bet it will be pricier than Sahl or El Gouna... I guess the delay is because they want to enter market in the right moment to be able to make the price higher.