According to the GRP (Greg's Race Parts) website listing (see link below) for one set of forged after-market rims sold by GRP, that will work for the Emira:
-the Emira OEM/stock rims weigh in at: Front=25.35 lbs; Rears=29.10 lbs
-this particular set of GRP rims weigh in at: Front=20.6 or 22.2 or 23.4 lbs (depending upon the style you choose); Rears=23 lbs
So that's a rear rim difference of (29.1 less 23) 6.1 lbs, times 2 rear rims = 12.2 lbs.
And a front rim difference of either (4.75 x 2) 9.5 lbs, or (3.15 x 2) 6.3 lbs or (1.95 x 2) 3.9 lbs.
The over-all unsprung weight savings, purchasing this particular set of rims from GRP (when combining both rear and front rim weight savings) ranges from (12.2 + 9.5) 21.7 lbs, to (12.2 + 6.3) 18.5 lbs, to (12.2 + 3.9) 16.1 lbs, depending upon the rim design chosen.
GRP Forged Monoblock Wheels for Evora & Emira! We set out to offer some new wheel options for the Evora and Emira but ran into some troubles locating a current wheel vendor/manufacturer who would work with us on custom sizes/offsets, lower volume, multiple designs choices and also not break...
www.gregsraceparts.com
Keep in mind that weights of other after-market rims vary, so what I've mentioned above, is just one example.
For another example and looking at the Aerie website, they mention a weight savings of 13.67 lbs (6.2 kg) on their V4 rims, compared to OEM: see link below:
Aerie Performant V4 Ultra-Light Forged Alloy Wheels Elevate your Lotus driving experience with Aerie Performant's NEW V4 Ultra-Light Forged Alloy Wheels. Engineered for models like the Lotus Emira, Electric, Evora, and Exige S3 V6, these wheels are designed to enhance both appearance and...
aerieperformance.com
Anyone looking for weight savings, has to decide how much he/she is willing to spend: good quality lighter rims will always cost more. Best to shop around and if you're prepared to wait, Black Friday or Boxing Day sale prices, usually offer considerable price reductions on rims and other accessories. This decision becomes more complex, when you're trying not only for weight savings, but focus on how the rim looks. Sometimes you can get both; other times you might sacrifice weight savings by buying a rim that looks better, in your opinion. Always choices to make.
Hope this answers your question and good luck with your decision.