Just my 2 cents worth.
Impiana is a UEM product and from my experience should be a good end product as compared to condo by other developer. 2 years back when I decided to gasak the Ujana unit, it is like the lower floor available but I went ahead anyway and today this has turn up well after 2 years and now it is ready and I think it is not a bad decision looking back.
As with investment, we are always speaking of current market valuation, you just have to make comparision with current market valuation of surrounding and comparable project to decide whether you are ok with the price.
I recently check out the new condo launch at Medini and they are alredy selling at $600 psf with 99 years lease. So newer projects are already selling at prices that factors in all the new mega project development. Impiana could be still at old prices with some increase.
I think the most important consideration is whether the unit that are available meets your requirements and preferences.
Also buy within your means and if the rental does not works out, you can always use for your own use and rent out your Singapore property.